delivered the opinion of the Court.
The Pattern Makers’ League of North America, AFL-CIO (League), a labor union, provides in its constitution that resignations are not permitted during a strike or when a strike is imminent. The League fined 10 of its members who, in violation of this provision, resigned during a strike and returned to work. The National Labor Relations Board held that these fines were imposed in violation of § 8(b)(1)(A) of the National Labor Relations Act, 29 U. S. C. § 158(b)(1)(A). We granted a petition for a writ of certiorari in order to decide whether § 8(b)(1)(A) reasonably may be construed by the *97Board as prohibiting a union from fining members who have tendered resignations invalid under the union constitution.
M
The League is a national union composed of local associations (locals). In May 1976, its constitution was amended to provide that
“[n]o resignation or withdrawal from an Association, or from the League, shall be accepted during a strike or lockout, or at a time when a strike or lockout appears imminent.”
This amendment, known as League Law 13, became effective in October 1976, after being ratified by the League’s locals. On May 6, 1977, when a collective-bargaining agreement expired, two locals began an economic strike against several manufacturing companies in Rockford, Illinois, and Beloit, Wisconsin. Forty-three of the two locals’ members participated. In early September 1977, after the locals formally rejected a contract offer, a striking union member submitted a letter of resignation to the Beloit Association.1 He returned to work the following day. During the next three months, 10 more union members resigned from the Rockford and Beloit locals and returned to work. On December 19, 1977, the strike ended when the parties signed a new collective-bargaining agreement. The locals notified 10 employees who had resigned that their resignations had been rejected as violative of League Law 13.2 The locals further informed the *98employees that, as union members, they were subject to sanctions for returning to work. Each was fined approximately the equivalent of his earnings during the strike.
The Rockford-Beloit Pattern Jobbers’ Association (Association) had represented the employers throughout the collective-bargaining process. It filed charges with the Board against the League and its two locals, the petitioners. Relying on § 8(b)(1)(A), the Association claimed that levying fines against employees who had resigned was an unfair labor practice.3 Following a hearing, an Administrative Law Judge found that petitioners had violated § 8(b)(1)(A) by fining employees for returning to work after tendering resignations. Pattern Makers’ League of North America, 265 N. L. R. B. 1332, 1339 (1982) (decision of G. Wacknov, ALJ). The Board agreed that § 8(b)(1)(A) prohibited the union from imposing sanctions on the 10 employees.4 Pattern Makers’ *99League of North America, supra. In holding that League Law 13 did not justify the imposition of fines on the members who attempted to resign, the Board relied on its earlier decision in Machinists Local 1327 (Dalmo Victor II), 263 N. L. R. B. 984 (1982), enf. denied, 725 F. 2d 1212 (CA9 1984).5
The United States Court of Appeals for the Seventh Circuit enforced the Board’s order. 724 F. 2d 57 (1983). The Court of Appeals stated that by restricting the union members’ freedom to resign, League Law 13 “frustrate^] the overriding policy of labor law that employees be free to choose whether to engage in concerted activities.” Id., at 60. Noting that the “mutual reliance” theory was given little weight in NLRB v. Textile Workers, 409 U. S. 213 (1972), the court rejected petitioners’ argument that their members, by participating in the strike vote, had “waived their Section 7 right to abandon the strike.” 724 F. 2d, at 60-61. Finally, the Court of Appeals reasoned that under Scofield *100v. NLRB, 394 U. S. 423 (1969), labor organizations may impose disciplinary fines against members only if they are “free to leave the union and escape the rule[s].” 724 F. 2d, at 61.
We granted a petition for a writ of certiorari, 469 U. S. 814 (1984), to resolve the conflict between the Courts of Appeals over the validity of restrictions on union members’ right to resign.6 The Board has held that such restrictions are invalid and do not justify imposing sanctions on employees who have attempted to resign from the union. Because of the Board’s “special competence” in the field of labor relations, its interpretation of the Act is accorded substantial deference. NLRB v. Weingarten, Inc., 420 U. S. 251, 266 (1975). The question for decision today is thus narrowed to whether the Board’s construction of § 8(b)(1)(A) is reasonable. See NLRB v. City Disposal Systems, Inc., 465 U. S. 822, 830 (1984). We believe that § 8(b)(1)(A) properly may be construed as prohibiting the fining of employees who have tendered resignations ineffective under a restriction in the union constitution. We therefore affirm the judgment of the Court of Appeals enforcing the Board’s order.
K
A
Section 7 of the Act, 29 U. S. C. § 157, grants employees the right to “refrain from any or all [concerted] . . . activities . . . .”7 This general right is implemented by *101§ 8(b)(1)(A). The latter section provides that a union commits an unfair labor practice if it “restraint] or coerce[s] employees in the exercise” of their § 7 rights.8 When employee members of a union refuse to support a strike (whether or not a rule prohibits returning to work during a strike), they are refraining from “concerted activity.” Therefore, imposing fines on these employees for returning to work “restraints]” the exercise of their §7 rights. Indeed, if the terms “refrain” and “restrain or coerce” are interpreted literally, fining employees to enforce compliance with any union rule or policy would violate the Act.
Despite this language from the Act, the Court in NLRB v. Allis-Chalmers Mfg. Co., 388 U. S. 175 (1967), held that § 8(b)(1)(A) does not prohibit labor organizations from fining current members. In NLRB v. Textile Workers, supra, and Machinists v. NLRB, 412 U. S. 84 (1973) (per curiam), the Court found as a corollary that unions may not fine former members who have resigned lawfully. Neither Textile Workers, supra, nor Machinists, supra, however, involved a provision like League Law 13, restricting the members’ right to resign. We decide today whether a union is precluded from fining employees who have attempted to resign when resignations are prohibited by the union’s constitution.9
*102B
The Court’s reasoning in Allis-Chalmers, supra, supports the Board’s conclusion that petitioners in this case violated § 8(b)(1)(A). In Allis-Chalmers, the Court held that imposing court-enforceable fines against current union members does not “restrain or coerce” the workers in the exercise of their §7 rights.10 In so concluding, the Court relied on the legislative history of the Taft-Hartley Act. It noted that the sponsor of § 8(b)(1)(A) never intended for that provision “ 'to interfere with the internal affairs or organization of unions,’” 388 U. S., at 187, quoting 93 Cong. Rec. 4272 (1947) (statement of Sen. Ball), and that other proponents of the measure likewise disclaimed an intent to interfere with unions’ “internal affairs.” 388 U. S., at 187-190. From the legislative history, the Court reasoned that Congress did not intend to prohibit unions from fining present members, as this was an internal matter. The Court has emphasized that the crux of Allis-Chalmers’ holding was the distinction between “internal and external enforcement of union rules . . . .” Scofield v. NLRB, 394 U. S., at 428. See also NLRB v. Boeing Co., 412 U. S. 67, 73 (1973).
The congressional purpose to preserve unions’ control over their own “internal affairs” does not suggest an intent to authorize restrictions on the right to resign. Traditionally, union members were free to resign and escape union discip*103line.11 In 1947, union constitutional provisions restricting the right to resign were uncommon, if not unknown.12 Therefore, allowing unions to “extend an employee's membership obligation through restrictions on resignation” would “expan[d] the definition of internal action” beyond the contours envisioned by the Taft-Hartley Congress. International Assn. of Machinists, Local 1414 (Neufeld Porsche-Audi, Inc.), 270 N. L. R. B. No. 209, p. 11 (1984).13
C
Language and reasoning from other opinions of this Court confirm that the Board’s construction of § 8(b)(1)(A) is rea*104sonable. In Scofield v. NLRB, supra, the Court upheld a union rule setting a ceiling on the daily wages that members working on an incentive basis could earn. The union members’ freedom to resign was critical to the Court’s decision that the union rule did not “restrain or coerce” the employees within the meaning of § 8(b)(1)(A). It stated that the rule was “reasonably enforced against union members who [were] free to leave the union and escape the rule.” Id., at 430. The Court deemed it important that if members were unable to take full advantage of their contractual right to earn additional pay, it was because they had “chosen to become and remain union members.” Id., at 435 (emphasis added).
The decision in NLRB v. Textile Workers, 409 U. S. 213 (1972), also supports the Board’s view that § 8(b)(1)(A) prohibits unions from punishing members not free to resign. There, 31 employees resigned their union membership and resumed working during a strike. We held that fining these former members “restrained or coerced” them, within the meaning of § 8(b)(1)(A). In reaching this conclusion, we said that “the vitality of § 7 requires that the member be free to refrain in November from the actions he endorsed in May.” Id., at 217-218. Restrictions on the right to resign curtail the freedom that the Textile Workers Court deemed so important. See also Machinists v. NLRB, 412 U. S. 84 (1973).
HI
Section 8(b)(1)(A) allows unions to enforce only those rules that “impai[r] no policy Congress has imbedded in the labor laws . . . .” Scofield, supra, at 430. The Board has found union restrictions on the right to resign to be inconsistent with the policy of voluntary unionism implicit in § 8(a)(3).14 *105See International Assn. of Machinists, Inc., Local 1)11 (Neufeld, Porsche-Audi, Inc.), supra; Machinists Local 1327 (Dalmo Victor II), 263 N. L. R. B., at 992 (Chairman Van de Water and Member Hunter, concurring). We believe that the inconsistency between union restrictions on the right to resign and the policy of voluntary unionism supports the Board’s conclusion that League Law 13 is invalid.
Closed shop agreements, legalized by the Wagner Act in 1935,15 became quite common in the early 1940’s. Under these agreements, employers could hire and retain in their employ only union members in good standing. R. Gorman, *106Labor Law, ch. 28, § 1, p. 639 (1976). Full union membership was thus compulsory in a closed shop; in order to keep their jobs, employees were required to attend union meetings, support union leaders, and otherwise adhere to union rules. Because of mounting objections to the closed shop, in 1947 — after hearings and full consideration — Congress enacted the Taft-Hartley Act. Section 8(a)(3) of that Act effectively eliminated compulsory union membership by outlawing the closed shop. The union security agreements permitted by § 8(a)(3) require employees to pay dues, but an employee cannot be discharged for failing to abide by union rules or policies with which he disagrees.16
Full union membership thus no longer can be a requirement of employment. If a new employee refuses formally to join a union and subject himself to its discipline, he cannot be fired. Moreover, no employee can be discharged if he initially joins a union, and subsequently resigns. We think it noteworthy that § 8(a)(3) protects the employment rights of the dissatisfied member, as well as those of the worker who never assumed full union membership. By allowing employees to resign from a union at any time, § 8(a)(3) protects the employee whose views come to diverge from those of his union.
League Law 13 curtails this freedom to resign from full union membership. Nevertheless, petitioners contend *107that League Law 13 does not contravene the policy of voluntary unionism imbedded in the Act. They assert that this provision does not interfere with workers’ employment rights because offending members are not discharged, but only fined. We find this argument unpersuasive, for a union has not left a “worker’s employment rights inviolate when it exacts [his entire] paycheck in satisfaction of a fine imposed for working. ” Wellington, Union Fines and Workers’ Rights, 85 Yale L. J. 1022, 1023 (1976). Congress in 1947 sought to eliminate completely any requirement that the employee maintain full union membership.17 Therefore, the Board was justified in concluding that by restricting the right of employees to resign, League Law 13 impairs the policy of voluntary unionism.
1 — 1 <3
We now consider specifically three arguments advanced by petitioners: (i) union rules restricting the right to resign are protected by the proviso to § 8(b)(1)(A); (ii) the legislative history of the Act shows that Congress did not intend to protect the right of union members to resign; and (iii) labor unions should be allowed to restrict the right to resign because other voluntary associations are permitted to do so.18
*108A
Petitioners first argue that the proviso to § 8(b)(1)(A) expressly allows unions to place restrictions on the right to resign. The proviso states that nothing in § 8(b)(1)(A) shall “impair the right of a labor organization to prescribe its own rules with respect to the acquisition or retention of membership therein.” 29 U. S. C. § 158(b)(1)(A). Petitioners contend that because League Law 18 places restrictions on the right to withdraw from the union, it is a “rul[e] with respect to the . . . retention of membership,” within the meaning of the proviso.19
Neither the Board nor this Court has ever interpreted the proviso as allowing unions to make rules restricting the right *109to resign.20 Rather, the Court has assumed that “rules "with respect to the . . . retention of membership” are those that provide for the expulsion of employees from the union.21 The legislative history of the Taft-Hartley Act is consistent with this interpretation. Senator Holland, the proviso’s sponsor, stated that § 8(b)(1)(A) should not outlaw union rules “which ha[ve] to do with the admission or the expulsion of members.” 93 Cong. Rec. 4271 (1947) (emphasis added). Senator Taft accepted the proviso, for he likewise believed that a union should be free to “refuse [a] man admission to the union, or expel him from the union” Id., at 4272 (emphasis added). Furthermore, the legislative history of the Labor-Management Reporting and Disclosure Act of 1959, 29 U. S. C. §401 et seq., confirms that the proviso was intended to protect union rules involving admission and expulsion.22 *110Accordingly, we find no basis for refusing to defer to the Board’s conclusion that League Law 13 is not a “rule with respect to the retention of membership,” within the meaning of the proviso.
B
The petitioners next argue that the legislative history of the Taft-Hartley Act shows that Congress made a considered decision not to protect union members’ right to resign. Section 8(c) of the House bill contained a detailed “bill of rights” for labor union members. H. R. 3020, § 8(c), 80th Cong., 1st Sess., 22-26 (1947). Included was a provision making it an unfair labor practice to “deny to any [union] member the right to resign from the organization at any time.” H. R. 3020, supra, § 8(c)(4), at 23. The Senate bill, on the other hand, did not set forth specific employee rights, but stated more generally that it was an unfair labor practice to “restrain or coerce” employees in the exercise of their §7 rights. H. R. 3020, 80th Cong., 1st Sess., §8(b)(1)(A), p. 81 (1947) (as passed by Senate). The Taft-Hartley Act contains the Senate bill’s general language rather than the more specific House prohibitions. See 29 U. S. C. § 158(b)(1)(A). The petitioners contend that the omission of the House provision shows that Congress expressly decided not to protect the “right to resign.”
The legislative history does not support this contention. The “right to resign” apparently was included in the original House bill to protect workers unable to resign because of “closed shop” agreements. Union constitutions limiting the right to resign were uncommon in 1947, see n. 12, supra; closed shop agreements, however, often impeded union resignations. The House Report, H. R. Rep. No. 245, 80th Cong., 1st Sess. (1947), confirms that closed shop agreements provided the impetus for the inclusion of a right to resign in *111the House bill. The Report simply states that even under the proposed legislation, employees could be required to pay dues pursuant to union security agreements. Id., at 32. Because the closed shop was outlawed by the Taft-Hartley Act, see § 8(a)(3), 29 U. S. C. § 158(a)(3), it is not surprising that Congress thought it unnecessary explicitly to preserve the right to resign.
Even if § 8(c)(4) of the House bill, H. R. 3020, supra, was directed at restrictive union rules, its omission from the Taft-Hartley Act does not convince us that the Board’s construction of § 8(b)(1)(A) is unreasonable. The House Conference Report, upon which petitioners primarily rely, does state that the specific prohibitions of § 8(c) were “omitted . . . as unfair labor practices,” H. R. Conf. Rep. No. 510, 80th Cong., 1st Sess., 46 (1947). But this language does not suggest that all employee rights arguably protected by the House bill were to be left unprotected. Cf. id., at 43 (“[T]he primary strike for recognition . . . was not prohibited”). Apparently, the Report was intended merely to inform House Members that the detailed prohibitions of § 8(c) were not separately included in the conference bill as “unfair labor practices.” We are reluctant to reach a contrary conclusion, and thereby overturn the Board’s decision, on the basis of this summary statement in the House Conference Report. Congress must have been aware that the broad language of § 8(b)(1)(A) would reach some of the same union conduct proscribed by the detailed “bill of rights.”23
*112Petitioners concede that “absent the legislative history,” the Board’s construction of § 8(b)(1)(A) would be entitled to deference. Tr. of Second Oral Arg. 15 (Apr. 1985). They argue, however, that “in this instance the legislative materials are too clearly opposed to what the Board did to permit the result the Board reached.” Id., at 17. We do not agree. The ambiguous legislative history upon which petitioners rely falls far short of showing that the Board’s interpretation of the Act is unreasonable.24
C
In Textile Workers, 409 U. S, at 216, and Machinists, 412 U. S., at 88 (per curiam), the Court stated that when a union constitution does not purport to restrict the right to resign, the “law which normally is reflected in our free institutions” is applicable. Relying on this quoted language, petitioners *113argue that League Law 13 is valid. They assert that because the common law does not prohibit restrictions on resignation,25 such provisions are not violative of § 8(b)(1)(A) of the Act. We find no merit in this argument. Textile Workers, supra, and Machinists, supra, held only that in the absence of restrictions on the right to resign, members are free to leave the union at any time. Although the Court noted that its decisions were consistent with the common-law rule, it did not state that the validity of restrictions on the right to resign should be determined with reference to common law.
The Court’s decision in NLRB v. Marine & Shipbuilding Workers, 391 U. S. 418 (1968), demonstrates that many union rules, although valid under the common law of associations, run afoul of § 8(b)(1)(A) of the Act.26 There the union ex*114pelled a member who failed to comply with a rule requiring the “exhaustion of] all remedies and appeals within the Union . . . before . . . resort to any court or other tribunal outside of the Union.” Id., at 421. Under the common law, associations may require their members to exhaust all internal remedies. See, e. g., Medical Soc. of Mobile Cty. v. Walker, 245 Ala. 135, 16 So. 2d 321 (1944). Nevertheless, the Marine Workers Court held that “considerations of public policy” mandated a holding that the union rule requiring exhaustion violated § 8(b)(1)(A), 29 U. S. C. § 158(b)(1)(A). 391 U. S., at 424; see also Scofield v. NLRB, 394 U. S., at 430 (union rule is invalid under § 8(b)(1)(A) if it “impairs [a] policy Congress has imbedded in the labor laws”).
The Board reasonably has concluded that League Law 13 “restrains or coerces” employees, see § 8(b)(1)(A), and is inconsistent with the congressional policy of voluntary unionism. Therefore, whatever may have been the common law, the Board’s interpretation of the Act merits our deference.
V
The Board has the primary responsibility for applying “ ‘the general provisions of the Act to the complexities of industrial life.’” Ford Motor Co. v. NLRB, 441 U. S. 488, 496 (1979), quoting NLRB v. Erie Resistor Corp., 373 U. S. 221, 236 (1963), in turn citing NLRB v. Steelworkers, 357 U. S. 357, 362-363 (1958). Where the Board’s construction of the Act is reasonable, it should not be rejected “merely because the courts might prefer another view of the statute.” Ford Motor Co. v. NLRB, supra, at 497. In this case, two factors suggest that we should be particularly reluctant to hold that the Board’s interpretation of the Act is impermissible. *115First, in related cases this Court invariably has yielded to Board decisions on whether fines imposed by a union “restrain or coerce” employees.27 Second, the Board consistently has construed § 8(b)(1)(A) as prohibiting the imposition of fines on employees who have tendered resignations invalid under a union constitution.28 Therefore, we conclude that the Board’s decision here is entitled to our deference.
f — t >
The Board found that by fining employees who had tendered resignations, the petitioners violated § 8(b)(1)(A) of the *116Act, even though League Law 13 purported to render the resignations ineffective. We defer to the Board’s interpretation of the Act and so affirm the judgment of the Court of Appeals enforcing the Board’s order.
It is so ordered.
William Kohl complained in his letter: “I no longer feel that the union officers are acting in the best interest of the men. We need fair and reasonable negotiators to solve our problems.” 3 Record, General Counsel Exhibit 2.
Kohl, the other employee who returned to work, was expelled from the union. On January 14, 1978, the Beloit local notified Kohl’s employer that because he was no longer a union member, he should be discharged pursuant to the “union shop” agreement. Two weeks later, the Beloit *98local informed Kohl that he could gain readmission to the union, and thus remain employed, if he paid back dues, a readmission fee, and $4,200 in “damages ... for deserting the strike by returning to work.” Pattern Makers’ League of North America, 265 N. L. R. B. 1332, 1337 (1982) (decision of G. Wacknov, ALJ). Kohl was denied readmission to the union because he refused to pay the amounts allegedly due. Nevertheless, he was not discharged by his employer. Ibid.
The Association also charged that petitioners committed the following unfair labor practices: (i) threatening employees with physical harm and loss of accrued pension benefits if they returned to work during the strike; and (2) attempting to have Kohl and another employee, John Nelson, discharged pursuant to a “union shop” agreement. Ibid. These charges are unrelated to the question presented in this case. Therefore, we express no opinion concerning the disposition of these claims by the ALJ, the Board, and the Court of Appeals.
Writing separately, Member Fanning asserted that a restriction on the right to resign would not violate § 8(b)(1)(A) if it functioned only to prevent members from removing their names from the union’s rolls. He agreed, however, that the employees could not be fined for returning to work after tendering their resignations. Pattern Makers’ League of North America, supra, at 1335 (Member Fanning, concurring and dissenting in part). Member Jenkins dissented, as he did in Machinist Local 1327 (Dalmo Victor II), 263 N. L. R. B. 984 (1982), enf. denied, 725 F. 2d 1212 (CA9 1984). See n. 5, infra.
In Machinists Local 1327 (Dalmo Victor II), several employees resigned from a union and returned to work during a strike. The union constitution prohibited resignations during, or within 14 days preceding, strikes. As in this case, the employees’ resignations were not accepted, and they were fined for aiding and abetting the employer. The Board held that fining these employees for returning to work after tendering resignations violated § 8(b)(1)(A).
Chairman Van de Water and Member Hunter stated that no restriction on the right to resign was permissible under the Act; they reasoned that such a rule allowed the union to exercise control over “external matters.” Moreover, these Board members thought that restrictions on resignation impaired the congressional policy, embodied in § 8(a)(3), of voluntary unionism. Therefore, they concluded that any discipline premised on such a rule violates § 8(b)(1)(A). 263 N. L. R. B., at 988.
Members Fanning and Zimmerman asserted that a rule legitimately could restrict the right to resign for a period of 30 days. Because the rule in question restricted the right to resign indefinitely, however, they agreed that the union had violated §8(b)(1)(A). Id., at 987.
Member Jenkins, the lone dissenter, contended that the union’s restriction on resignation was protected by the proviso to § 8(b)(1)(A), which states that a union may “prescribe its own rules with respect to the acquisition or retention of membership therein.” Id., at 993.
The Court of Appeals for the Ninth Circuit has held that a union may impose restrictions on its members’ right to resign. Machinists Local 1327 (Dalmo Victor II), supra. See n. 5, supra, for a discussion of the Board decision that the Court of Appeals for the Ninth Circuit refused to enforce.
Section 7 of the Act, as set forth in 29 U. S. C. § 157, provides:
“Employees shall have the right to self-organization, to form, join, or assist labor organizations, to bargain collectively through representatives of their own choosing, and to engage in other concerted activities for the purpose of collective bargaining or other mutual aid or protection, and shall *101also have the right to refrain from any or all of such activities except to the extent that such right may be affected by an agreement requiring membership in a labor organization as a condition of employment as authorized in section 158(a)(3) of this title.”
Section 8(b)(1)(A), as set forth in 29 U. S. C. § 158(b)(1)(A), provides:
“It shall be an unfair labor practice for a labor organization or its agents—
“(1) to restrain or coerce (A) employees in the exercise of the rights guaranteed in section 157 of this title: Provided, That this paragraph shall not impair the right of a labor organization to prescribe its own rules with respect to the acquisition or retention of membership therein . . . .”
In both Textile Workers, 409 U. S., at 217, and Machinists, 412 U. S., at 88, the Court explicitly left open the question of “the extent to which *102contractual restriction on a member’s right to resign may be limited by the Act.” Ibid.
The proviso to § 8(b)(1)(A), 29 U. S. C. § 158(b)(1)(A), states that nothing in the section shall “impair the right of a labor organization to prescribe its own rules with respect to the acquisition or retention of membership therein.” The Court in Allis-Chalmers assumed that the proviso could not be read to authorize the imposition of court-enforceable fines. 388 U. S., at 192. See NLRB v. Boeing Co., 412 U. S. 67, 71, n. 5 (1973) (“This Court... , in holding that court enforcement of union fines was not an unfair labor practice in NLRB v. Allis-Chalmers Mfg. Co., relied on congressional intent only with respect to the first part of this section”) (citation omitted).
See Bossert v. Dhuy, 221 N. Y. 342, 365, 117 N. E. 582, 587 (1917) (“The members of the organization . . . who are not willing to obey the orders of the organization are at liberty to withdraw therefrom”); Barker Painting Co. v. Brotherhood of Painters, 57 App. D. C. 322, 324, 23 F. 2d 743, 745, cert. denied, 276 U. S. 631 (1928) (It is “not unlawful for . . . unions to punish a member by fine, suspension, or expulsion for an infraction of the union rules, since membership in the union is purely voluntary”); Bayer v. Brotherhood of Painters, Decorators and Paperhangers of America, Local 301, 108 N. J. Eq. 257, 262, 154 A. 759, 761 (1931) (“association is a voluntary one, and the workmen may decline to become members or withdraw from membership, if dissatisfied with the conduct of its affairs”); Mische v. Kaminski, 127 Pa. Super. 66, 91-92, 193 A. 410, 421 (1937) (members “had a right to leave the union”); Longshore Printing Co. v. Howell, 26 Ore. 527, 540, 38 P. 547, 551 (1894) (“No resort can be had to compulsory methods of any kind either to increase, keep up, or retain such membership”).
Our attention has not been called to any provision limiting the right to resign in a union constitution extant in 1947. Indeed, even by the 1970’s, very few unions had such restrictions in their constitutions. See Millan, Disciplinary Developments Under § 8(b)(1)(A) of the National Labor Relations Act, 20 Loyola L. Rev. 245, 269 (1974); Wellington, Union Fines and Workers’ Rights, 85 Yale L. J. 1022, 1042 (1976).
In International Assn. of Machinists, Local 1414 (Neufeld Porsche-Audi, Inc.), a majority of the Board held that any restriction on the right to resign violates the Act. This was the position taken by Chairman Van de Water and Member Hunter in Machinists Local 1327 (Dalmo Victor II), 263 N. L. R. B. 984 (1982), enf. denied, 725 F. 2d 1212 (CA9 1984). See n. 5, supra.
Section 8(a)(3), as set forth in 29 U. S. C. § 158(a)(3), provides: “It shall be an unfair labor practice for an employer—
“(3) by discrimination in regard to hire or tenure of employment or any term or condition of employment to encourage or discourage membership *105in any labor organization: Provided, That nothing in this subchapter, or in any other statute of the United States, shall preclude an employer from making an agreement with a labor organization ... to require as a condition of employment membership therein on or after the thirtieth day following the beginning of such employment or the effective date of such agreement, whichever is the later...: Provided further, That no employer shall justify any discrimination against an employee for nonmembership in a labor organization (A) if he has reasonable grounds for believing that such membership was not available to the employee on the same terms and conditions generally applicable to other members, or (B) if he has reasonable grounds for believing that membership was denied or terminated for reasons other than the failure of the employee to tender the periodic dues and the initiation fees uniformly required as a condition of acquiring or retaining membership.”
Section 8(b)(2) of the Act, as set forth in 29 U. S. C. § 158(b)(2), complements § 8(a)(3) by providing that:
“It shall be an unfair labor practice for a labor organization or its agents—
“(2) to cause or attempt to cause an employer to discriminate against an employee in violation of subsection (a)(3) of this section or to discriminate against an employee with respect to whom membership in such organization has been denied or terminated on some ground other than his failure to tender the periodic dues and the initiation fees uniformly required as a condition of acquiring or retaining membership.”
Section 8(3) of the Wagner Act, ch. 372, 49 Stat. 452, generally barred discrimination based on union membership. A proviso to that section stated, however, that “nothing in this Act. . . shall preclude an employer from making an agreement with a labor organization ... to require as a condition of employment membership therein . . . .”
Under § 8(a)(3), the only aspect of union membership that can be required pursuant to a union shop agreement is the payment of dues. See Radio Officers v. NLRB, 347 U. S. 17, 41 (1954) (union security agreements cannot be used for “any purpose other than to compel payment of union dues and fees”). “ ‘Membership,’ as a condition of employment, is whittled down to its financial core.” NLRB v. General Motors Corp., 373 U. S. 734, 742 (1963). See also Ellis v. Railway Clerks, 466 U. S. 435 (1984) (under the Railway Labor Act, employees in a “union shop” cannot be compelled to pay dues to support certain union activities). Therefore, an employee required by a union security agreement to assume financial “membership” is not subject to union discipline. Such an employee is a “member” of the union only in the most limited sense.
The focus of § 8(a)(3) on employment rights is understandable because union restrictions on the right to resign were not an issue in 1947. See n. 12, supra, and accompanying text. Senator Taft, for example, stated that § 8(a)(3) was designed to prevent the discharge of workers for reasons other than nonpayment of dues, 93 Cong. Ree. 4885-4886 (1947), because this was “the usual type of abuse, and is the only type of abuse testified to.” Id., at 4886.
The dissent suggests that the Board’s decision is inconsistent with 29 U. S. C. § 163, which provides that nothing in the Act “shall be construed so as ... to interfere with or impede or diminish in any way the right to strike.” The Board does not believe, and neither do we, that its interpretation of § 8(b)(1)(A) impedes the “right to strike.” “It [will] not outlaw anybody striking who want[s] to strike. It [will] not prevent anyone using the strike in a legitimate way .... All it [will] do [is] . . . outlaw such *108restraint and coercion as would prevent people from going to work if they wished to go to work.” 93 Cong. Rec. 4436 (1947) (remarks of Sen. Taft).
Moreover, we do not believe that the effectiveness of strikes will be unduly hampered by the Board’s decision. An employee who voluntarily has joined a union will be reluctant to give up his membership. As Dean Wellington has said:
“In making his resignation decision, the dissident must remember that the union whose policies he finds distasteful will continue to hold substantial economic power over him as exclusive bargaining agent. By resigning, the worker surrenders his right to vote for union officials, to express himself at union meetings, and even to participate in determining the amount or use of dues he may be forced to pay under a union security clause.” Wellington, Union Fines and Workers’ Rights, 85 Yale L. J. 1022, 1046 (1976).
Justice Blackmun’s dissent asserts that League Law 13 is protected by the proviso because the rule “literally involvfes] the acquisition and retention of membership.” Post, at 121. This interpretation of the proviso would authorize any union restriction on the right to resign. The dissent does say that restrictions on resignation would not be permitted if they “furthered none of the purposes of collective action and self-organization.” Post, at 132, n. 5. This limitation is illusory. An absolute restriction on resignations would enhance a union’s collective-bargaining power, as would a rule that prohibited resignations during the life of the collective-bargaining agreement. In short, there is no limiting principle to the dissent’s reading of the proviso.
Justice Blackmun’s dissent also interprets the proviso in a novel manner. He asserts that the only union rules prohibited by § 8(b)(1)(A) are those which “seek to coerce an employee by utilizing the employer’s power over his employment status, or otherwise compel him to take on duties or join in concerted activities he never consented to.” Post, at 120. This position is inconsistent with the Court’s holding in NLRB v. Marine & Shipbuilding Workers, 391 U. S. 418 (1968). In that case, the Court held invalid a union rule requiring the “exhaustion of] all remedies and appeals within the Union . . . before . . . resort to any court or other tribunal outside of the Union.” Id., at 421. The rule was enforced by the imposition of fines, without “utilizing the employer’s power over the violating members’ employment status.” Moreover, there was no suggestion that union members had not voluntarily agreed to be bound by the rule requiring exhaustion.
In NLRB v. Allis-Chalmers Mfg. Co., 388 U. S., at 192, the Court assumed that the proviso authorized unions to expel members and to impose fines that carry the threat of expulsion.
During the House debates on the Labor-Management Reporting and Disclosure Act of 1959, 29 U. S. C. § 401 et seq., it was proposed that the pending bill be amended to prohibit unions from expelling members for discriminatory reasons. 105 Cong. Rec. 15721 (1959). The two sponsors of the bill, Representatives Landrum and Griffin, opposed this amendment on the ground that they did not seek to repeal the proviso to § 8(b)(1)(A). Id., at 15722-15723. As this Court observed in NLRB v. Drivers, 362 *110U. S. 274, 291 (1960): “[WJhat Congress did in 1969 does not establish what it meant in 1947. However, as another major step in an evolving pattern of regulation of union conduct, the 1959 Act is a relevant consideration.”
Section 8(c)(9) of the House bill, H. R. 3020, 80th Cong., 1st Sess., 25, prohibited unions from “intimidat[ing a member’s] family.” Although this specific provision was omitted from the Taft-Hartley Act, it is unlikely that Congress intended to protect all union conduct that § 8(c)(9) would have proscribed. Union threats against the family of a member who crossed a picket line, for example, would seem to “restrain or coerce” him in the exercise of his § 7 rights.
The dissent by Justice Blackmun suggests that because the Senate “explicitly has rejected” the specific prohibitions in § 8(c) of the House bill, see post, at 121,122, the Taft-Hartley Act leaves unregulated the relation*112ship between a union and its members. The legislative history shows, however, that the Senate did not intend such a result. Senator Taft stated that § 8(b)(1)(A) was designed to warn unions that “they do not have the right to interfere with or coerce employees, either their own members or those outside their union.” 93 Cong. Rec. 4025 (1947) (emphasis added).
NLRB v. Drivers, 362 U. S. 274 (1960), is not controlling. There the Court held that recognitional picketing did not “restrain or coerce” employees in violation of § 8(b)(1)(A), 29 U. S. C. § 158(b)(1)(A). In so concluding, the Court relied in part on the omission from the Taft-Hartley Act of two provisions in the original House bill explicitly banning recognitional picketing. Other evidence in that case, however, was highly probative of a congressional intent to protect recognitional picketing. The desirability of restrictions on peaceful picketing was widely debated in Congress. Id., at 287-288. Moreover, the Conference Report stated that “ ‘the primary strike for recogition . . . was not prohibited.’” Id., at 289, quoting H. R. Conf. Rep. No. 510, 80th Cong., 1st Sess., 43 (1947). Finally, §8(b)(4), as added by the Taft-Hartley Act, 29 U. S. C. § 158(b)(4), limits the use of other economic weapons (such as strikes and secondary boycotts) to compel recognition by an employer. See 362 U. S., at 282-284. It is clear that a compromise was reached by the 1947 Congress; recognitional picketing was to be allowed, while the use of other economic weapons to compel recognition was curtailed. There is no evidence of such a compromise with respect to the “right to resign.”
It is at least open to question whether all restrictions on the right to resign are valid under the “common law of associations.” See, e. g., Haynes v. Annandale Golf Club, 4 Cal. 2d 28, 47 P. 2d 470 (1935) (a bylaw purporting to allow the association to deny a member’s right of resignation by merely withholding its consent is invalid because unreasonable and arbitrary). Our conclusion that the common law is irrelevant makes it unnecessary to resolve this question.
Justice Blackmun’s dissent suggests that the relationship between a union and its members should be governed by contract law. Post, at 119. The rationale of this theory is that a member, by joining the union, “enters into a contract, the terms of which are expressed in the union constitution and by-laws.” Summers, Legal Limitations on Union Discipline, 64 Harv. L. Rev. 1049, 1054 (1951). Marine Workers shows, of course, that union discipline cannot be analyzed primarily in terms of the common law of contracts.
The dissent repeatedly refers to the “promise” made by the employees involved in this case. Post, at 129. Because they were members of the union when League Law 13 was adopted, the dissent reasons that the employees “promised” not to resign during a strike. But the “promise” to which the dissent refers is unlike any other in traditional contract law. As a commentator has recognized:
“Membership in a union contemplates a continuing relationship with changing obligations as the union legislates in monthly meetings or in annual conventions. It creates a complex cluster of rights and duties *114expressed in a constitution. In short, membership is a special relationship. It is as far removed from the main channel of contract law as the relationships created by marriage, the purchase of a stock certificate, or the hiring of a servant.”
Summers, supra, at 1055-1056.
In holding that unions may impose fines against members who return to work during a strike, the Court in NLRB v. Allis-Chalmers Mfg. Co., 388 U. S. 175 (1967), “essentially accepted the position of the National Labor Relations Board.” Scofield v. NLRB, 394 U. S. 423, 428 (1969). In Scofield, the Court deferred to the Board’s ruling that § 8(b)(1)(A) does not prohibit unions from fining members who accept daily wages in excess of a union-imposed ceiling. Four years later, the Court again relied on the Board in holding that § 8(b)(1)(A) has nothing to say about whether union fines are “reasonable” in amount. NLRB v. Boeing Co., 412 U. S. 67 (1973). Finally, in NLRB v. Textile Workers, 409 U. S. 213 (1972), and Machinists v. NLRB, 412 U. S. 84 (1973) (per curiam), the Court deferred to the Board’s conclusion that § 8(b)(1)(A) prohibits unions from fining former members who have resigned lawfully.
In United Automobile, Aerospace & Agricultural Implement Workers, Local 617 (General Electric Co.), 197 N. L. R. B. 608 (1972), the Board held that § 8(b)(1)(A) prohibits a union from fining employees who have resigned, even when a provision in the union constitution purports to make the resignations invalid. There two employees resigned during a strike and returned to work. Their resignations were ineffective under a union constitutional provision permitting resignations only during the last 10 days of the union’s fiscal year. The Board nevertheless held that the employees could not be fined for crossing the picket line. It noted that imposing fines on these employees was inconsistent with Scofield v. NLRB, supra, for they effectively were denied “a voluntary method of severing their relationship with the Union.” 197 N. L. R. B., at 609. The Board reached the same conclusion in United Automobile, Aerospace & Agricultural Implement Workers, Local 4-69 (Master Lock Co.), 221 N. L. R. B. 748 (1975). See also Local 1884, United Automobile, Aerospace & Agricultural Implement Workers (Ex-Cell-O Corp.), 219 N. L. R. B. 729 (1975).