Opinion
CHIN, J.State law imposes an annual registration requirement on certain sex offenders while they reside or are located in California. (Pen. Code, § 290, subd. (a)(1)(A), (C).)1 Offenders subject to registration must also notify authorities of any change of their address. (§ 290, subd. (f)(1).) A 1998 amendment to section 290, adding subdivision (f)(1), specifically requires offenders to give change-of-address notification on leaving California. (Stats. 1998, ch. 930, § 1.1.)
Defendant in the present case failed to notify the California authorities with whom he had initially registered under section 290 that he and his wife *251had moved from California to Texas on May 1, 1995. The trial court found him guilty of violating the notification provisions of the state registration law then in effect (§ 290, former subd. (f)), and also found true two prior “strike” allegations (§§ 667, subds. (b)-(i), 1170.12, subds. (a)-(d)), arising out of two sex offenses he committed in 1985. Accordingly, the court sentenced defendant to state prison for 25 years to life for failing to comply with the registration statute. The Court of Appeal affirmed the judgment. We granted review.
We consider whether the notification requirement in effect when defendant moved to Texas applied to persons who left the state to live elsewhere. As will appear, we conclude that, before the amendment of the registration statute in 1998, it was ambiguous as to its application to departing residents such as defendant. Accordingly, we must construe the ambiguous statute in defendant’s favor and will reverse the decision of the Court of Appeal affirming his conviction.
Section 290
Under the registration provision applicable to defendant, a sex offender was required “for the rest of his . . . life while residing in California, . . . to register . . . within 14 days of coming into any county [or] city ... in which he . . . temporarily resides or is domiciled for that length of time.” (§ 290, former subd. (a)(1), as amended by Stats. 1994, ch. 867, § 2.7; see § 290, present subd. (a)(1)(A) [registration within five working days].) Language limiting section 290 to California residents was added in 1994. (§ 290, former subd. (a)(1), as amended by Stats. 1994, ch. 867, § 2.7.) At the same time, violation of the statute became a felony for registrants previously convicted of felony sex crimes. (§ 290, former subd. (g)(2), as amended by Stats. 1994, ch. 867, § 2.7.) Persons subject to section 290 were required to inform the appropriate law enforcement agency of any change of “residence address” within 10 days. (§ 290, former subd. (f), as amended by Stats. 1994, ch. 867, § 2.7; see § 290, present subd. (f)(1) [notice within five working days].) Under both versions of the registration statute, the agency is required to forward changed address information to the Department of Justice, which in turn must forward it to the appropriate agencies in the new place of residence. (§ 290, former subd. (f), as amended by Stats. 1994, ch. 867, § 2.7; see present § 290, subd. (f)(1).) Likewise, under both versions, willful violation of the registration or notification provisions by a person initially required to register because he committed a prior felony is itself a felony offense. (§ 290, former subd. (g)(2), as amended by Stats. 1994, ch. 867, § 2.7; see present § 290, subd. (g)(2).)
A 1998 amendment to section 290, subdivision (f)(1), adopted after we granted review in this case, and effective January 1, 1999, specifically *252requires change of address notification when a registrant “changes his . . . residence address or location, whether within the jurisdiction in which he ... is currently registered or to a new jurisdiction inside or outside the state . . . .” (Stats. 1998, ch. 930, § 1.1, italics added.) Although the amendment makes it clear that in the future registrants must give a change-of-address notification when leaving California, courts confronted with violations of earlier versions of section 290 may benefit from our clarification of the issue.
Facts
The following uncontradicted facts are largely taken from the Court of Appeal opinion in this case. Defendant was subject to the sex offender registration law as a result of his 1985 convictions of child molestation and child sodomy. He first registered in 1989, but evidently failed to register annually after that time. (Defendant’s conviction was not based, however, on his failure to register, an offense of which he was acquitted.) In late 1994 and early 1995, defendant lived in North Hollywood and Wilmington, California. On May 1, 1995, defendant and his wife moved to Texas. Defendant failed to notify any California law enforcement agency of any of his various address changes. In December 1995, defendant was arrested in Texas and then returned to California.
Defendant was charged with two counts of child molestation (§ 288, subd. (a)), two violations of the sex offender registration law (§ 290, former subds. (a)(1) [failure to re-register in June 1995], (f) [failure to notify of changed address from January 1995 through December 1996], (g)(2) [making these violations felony offenses]), and various felony conviction enhancements and “strikes.” Defendant waived jury trial, in return for which the prosecution agreed to dismiss both molestation charges. Defendant did not testify or present any evidence at trial.
The trial court acquitted defendant of the failure to re-register count but convicted him of the other registration act count, based solely on his failure to notify state authorities of his move to Texas. The court also found true both “strikes.” Defendant was subject to the sex offender registration law as a result of his 1985 felony convictions of child molestation and child sodomy. Accordingly, his present offense was likewise a felony. (§ 290, subd. (g)(2).) As previously noted, the court sentenced defendant to state prison for 25 years to life.
The Court of Appeal Decision
The Court of Appeal would have affirmed the judgment, reasoning as follows: “The duty to register and give notice of address changes is a *253continuing one. (Wright v. Superior Court (1997) 15 Cal.4th 521 [63 Cal.Rptr. 322, 936 P.2d 101].) As our Supreme Court has observed: ‘Plainly, the Legislature perceives that sex offenders pose a “continuing threat to society” [citation] and require constant vigilance.’ (Id. at p. 527.) [ft] We hold that ‘while residing in California’ (Pen. Code, § 290, [former] subd. (a)(1)) applies to a person who changes his residence from California to a residence outside California. When [defendant] departed his California residence he did not yet reside in Texas. The departure, however, created a notification of address change duty. Had [defendant] fulfilled this duty, established residency in Texas, and then moved again (but not back to California) he could properly make the argument he now asserts: He was not ‘residing in California.’ ”
Discussion
We disagree with the Court of Appeal’s overly technical analysis. Initially, we note that failure to comply with California’s sex offender registration law constitutes a penal offense. (§ 290, subd. (g).) In the present case, defendant was convicted of a felony for failing to comply with the statute and was sentenced to 25 years to life in prison.2 That being so, the statute must be construed as favorably to the defendant as its language and the circumstances of its application reasonably may permit. (People v. McCleod (1997) 55 Cal.App.4th 1205, 1216 [64 Cal.Rptr.2d 545]; see People ex rel. Lungren v. Superior Court (1996) 14 Cal.4th 294, 312 [58 Cal.Rptr.2d 855, 926 P.2d 1042]; In re Christian S. (1994) 7 Cal.4th 768, 780 [30 Cal.Rptr.2d 33, 872 P.2d 574]; Keeler v. Superior Court (1970) 2 Cal.3d 619, 631 [87 Cal.Rptr. 481, 470 P.2d 617, 40 A.L.R.3d 420]; cf. Lambert v. California (1957) 355 U.S. 225, 229 [78 S.Ct. 240, 243, 2 L.Ed.2d 228] [“[A]ctual knowledge of the duty to register or proof of the probability of such knowledge and subsequent failure to comply are necessary before a conviction under the [sex offender registration] ordinance can stand.”].)
This principle of favorable construction is especially apposite to registration statutes, which, to assure effective compliance, must give clear notice to all registrants of their responsibilities so that laypersons such as defendant can readily understand and properly discharge them. (See Cal. Const., art. I, § 7 [due process clause]; Walker v. Superior Court (1988) 47 Cal.3d 112, 143 [253 Cal.Rptr. 1, 763 P.2d 852] [“ ‘A statute should be sufficiently *254certain so that a person may know what is prohibited thereby and what may be done without violating its provisions . . . ”]; Grayned v. City of Rochford (1972) 408 U.S. 104, 108 [92 S.Ct. 2294, 2298-2299, 33 L.Ed.2d 222] [Laws must “give the person of ordinary intelligence a reasonable opportunity to know what is prohibited, so that he may act accordingly. Vague laws may trap the innocent by not providing fair warning.”].)
Defendant argues that, under one reasonable interpretation of the version of section 290 applicable to him, his duty to register annually with California law enforcement agencies continued only while he resided in this state. Assuming his registration duty terminated when he left the state, his related obligation to notify authorities regarding his new address would also have terminated. Former subdivision (f) of section 290, containing the notification provision, applied only to a “person who is required to register pursuant to this section.”
This construction of the statute has a rational basis: Although the state has a clear and legitimate interest in protecting its own citizens from criminal offenders by tracking their whereabouts within its borders, this interest is arguably diminished when offenders leave this state to establish residence elsewhere.
As we recently stated in Wright v. Superior Court (1997) 15 Cal.4th 521, 527 [63 Cal.Rptr.2d 322, 936 P.2d 101], the registration act is intended to promote the state’s interest in controlling crime and preventing recidivism in sex offenders by making them readily available for police surveillance at all times. (See also People v. McClellan (1993) 6 Cal.4th 367, 376, fn. 7 [24 Cal.Rptr.2d 739, 862 P.2d 739], and cases cited.) But California’s interest in controlling crime and surveilling prior offenders is strongest within its borders. Once registrants leave California, the states they enter assume primary responsibility for surveilling them. This principle is illustrated by statutes in sister states, including Texas, imposing registration and address change notification duties on sex offenders coming from other states. (See, e.g., Tex. Code Crim. P. Ann. arts. 62.01-62.02 (West 1998) [registration requirement], 62.04 [address change notification].) Indeed, the California registration act now requires that all registrants be notified that “he or she may have a duty to register in any other state where he or she may relocate.” (§ 290, subd. (e)(2)(D), added by Stats. 1997, ch. 821, § 3.5.)
Significantly, several other states with registration statutes similar to California’s explicitly require offenders to notify state officials when they leave the state. For example, under the Texas statute, offenders who intend to change their address or to move to another state must report to the local *255law enforcement agency with whom they last registered and provide the anticipated move date and new address. (Tex. Code Crim. P. Ann., supra, art. 62.04(a).) An offender who moves to another state that has a sex offender registration statute must register with that state’s law enforcement agency within 10 days of arrival. (Tex. Code Crim. P. Ann., supra, art. 62.04(c).)
Similar provisions exist in the sex offender registration statutes of Arkansas (Ark. Code Ann. § 12-12-909, subd. (c)(1) (Michie 1987, 1997 supp.)), Georgia (Ga. Code Ann. § 42-1-12, subd. (b)(3)(A)(iii) (1998)), Hawaii (Haw. Rev. Stat. § 846E-7 (1998)), Illinois (111. Comp. Stat. Ann. ch. 730, act 150, § 6 (West 1998)), Iowa (Iowa Code Ann. § 692A.3, subd. 4 (West 1998)), Louisiana (La. Rev. Stat. Ann. § 542.1C(3) (West 1998)), Massachusetts (Mass. Gen. Laws Ann. ch. 6, § 178E(e) (West 1998)), Nevada (Nev. Rev. Stat. Ann. § 179D.470, subd. 2(b) (Michie 1997)), Ohio (Ohio Rev. Code Ann. § 2950.05(C) (Banks-Baldwin 1998)), South Carolina (S.C. Code Ann. § 23-3-460 (Law. Co-op. 1998)), Vermont (Vt. Stat. Ann. tit. 13, § 5407, subd. (b) (1998)), Washington (Wash. Rev. Code Ann. § 9A.44.130, subd. (5)(a) (West 1998)), and West Virginia (W. Va. Code § 61-8F-7 (1998)), each of which by its terms requires registered offenders to notify state authorities when they leave the state. As noted, until 1998, the California registration statute did not expressly require this form of notice. Although the parties have not submitted any conclusive legislative history, the 1998 amendment may reflect the Legislature’s recognition that the former provision was ambiguous in this regard.
We recognize that defendant’s interpretation of former section 290 is not the only possible reading of the statute. One reasonably could accept the People’s contrary view, which the Court of Appeal adopted, that (1) when defendant departed for Texas, he nonetheless temporarily continued to “reside” in California (see Gov. Code, § 244, subd. (c)) and therefore remained subject to the registration act’s notification provisions, and (2) California has a continuing interest in tracking sex offenders outside state boundaries, to assure continuing surveillance and notification to interested authorities in other states.
But given the limited application of section 290, former subdivision (a)(1), to offenders “while residing in California,” its provisions were at least ambiguous regarding their application to persons, such as defendant, who move to another state. Certainly, a layperson such as defendant could reasonably assume that the 1995 version of the act did not apply to him once he left California. Accordingly, we must apply the rule, discussed above, that any statutory ambiguities in a penal law ordinarily should be construed in the defendant’s favor.
*256“[T]he determination of the meaning of statutes is a judicial function . . . (People v. Cruz (1996) 13 Cal.4th 764, 781 [55 Cal.Rptr.2d 117, 919 P.2d 731].) We look to all pertinent circumstances and considerations in deciding- whether an amendment is a modification or clarification of a statute. (Ibid.) Nothing in section 290, before its 1998 amendment, conclusively establishes that the statute imposed notification requirements on nonresidents. Change-of-address notification was required within 10 days of moving, which in this case occurred after defendant established residency in Texas. Under one reasonable interpretation of former section 290, at that point defendant, as a nonresident, had no further responsibilities under the California statute.
We note that defendant may have violated section 290, former subdivision (f), by failing to notify California authorities of his earlier change of addresses within California. Defendant’s conviction was not based, however, on this failure to notify. The trial court’s reliance on a legally incorrect theory of guilt requires that we reverse defendant’s conviction. We express no opinion as to whether the evidence is sufficient to convict defendant of violating section 290, subdivision (f), under the “continuing offense” theory approved in Wright v. Superior Court, supra, 15 Cal.4th 521, 527-528.
The judgment of the Court of Appeal is reversed.
George, C. J., Mosk, J., Kennard, J., and Werdegar, J., concurred.
Further statutory references are to the Penal Code unless otherwise indicated.
We express no opinion on whether the obligation that a person register as a sex offender, imposed as a consequence of a conviction of a criminal offense, constitutes punishment for purposes of ex post facto analysis. That issue is before us in People v. Castellanos (Cal.App.) review granted Nov. 25, 1997 (S064388).