Syllabus he can be detained pursuant to a single judicial proceeding is one year. The State's use of procedural safeguards applicable in criminal trials does not itself turn the proceedings into criminal prosecutions. Allen, supra, at 372. Finally, the Act is not necessarily punitive if it fails to offer treatment where treatment for a condition is not possible, or if treatment, though possible, is merely an ancillary, rather than an overriding, state concern. The conclusion that the Act is nonpunitive removes an essential prerequisite for both Hendricks' double jeopardy and ex post facto claims. Pp. 360-369. (b) Hendricks' confinement does not amount to a second prosecution and punishment for the offense for which he was convicted. Because the Act is civil in nature, its commitment proceedings do not constitute a second prosecution. Cf. Jones, supra. As this commitment is not tantamount to punishment, the detention does not violate the Double Jeopardy Clause, even though it follows a prison term. Baxstrom v. Herold, 383 U. S. 107. Hendricks' argument that, even if the Act survives the "multiple punishments" test, it fails the "same elements" test of Blockburger v. United States, 284 U. S. 299, is rejected, since that test does not apply outside of the successive prosecution context. Pp. 369-370. (c) Hendricks' ex post facto claim is similarly flawed. The Ex Post Facto Clause pertains exclusively to penal statutes. California Dept. of Corrections v. Morales, 514 U. S. 499, 505. Since the Act is not punishment, its application does not raise ex post facto concerns. Moreover, the Act clearly does not have retroactive effect. It does not criminalize conduct legal before its enactment or deprive Hendricks any defense that was available to him at the time of his crimes. Pp. 370-371. of 259 Kan. 246, 912 P. 2d 129, reversed. THOMAS, J., delivered the opinion of the Court, in which REHNQUIST, C. J., and O'CONNOR, SCALIA, and KENNEDY, JJ., joined. KENNEDY, J., filed a concurring opinion, post, p. 371. BREYER, J., filed a dissenting opinion, in which STEVENS and SOUTER, JJ., joined, and in which GINSBURG, J., joined as to Parts II and III, post, p. 373. Carla J. Stovall, Attorney General of Kansas, argued the cause for the petitioner in No. 95-1649 and respondent in No. 95-9075. With her on the briefs were Stephen R. McAllister, Special Assistant Attorney General, Bernard Nash, James van R. Springer, and Laura B. Feigin. Counsel Thomas J. Weilert argued the cause for Hendricks in both cases. With him on the briefs were James W. Ellis and David Gottlieb.† †Briefs of amici curiae urging reversal were filed for the State of Washington et al. by Christine O. Gregoire, Attorney General of Washington, and Sarah Blackman Sappington, Assistant Attorney General, Charles F. C. Ruff, Corporation Counsel of the District of Columbia, Sebastian Aloot, Acting Attorney General of the Northern Mariana Islands, and by the Attorneys General for their respective jurisdictions as follows: Jeff Sessions of Alabama, Malaetasi Togafu of American Samoa, Grant Woods of Arizona, Winston Bryant of Arkansas, Daniel E. Lungren of California, Gale A. Norton of Colorado, M. Jane Brady of Delaware, Robert A. Butterworth of Florida, Calvin E. Holloway, Sr., of Guam, Margery S. Bronster of Hawaii, Alan G. Lance of Idaho, James E. Ryan of Illinois, Pamela Fanning Carter of Indiana, Thomas J. Miller of Iowa, A. B. Chandler III of Kentucky, Richard P. Ieyoub of Louisiana, J. Joseph Curran, Jr., of Maryland, Hubert H. Humphrey III of Minnesota, Mike Moore of Mississippi, Jeremiah W. (Jay) Nixon of Missouri, Joseph P. Mazurek of Montana, Don Stenberg of Nebraska, Frankie Sue Del Papa of Nevada, Jeffrey R. Howard of New Hampshire, Peter Verniero of New Jersey, Tom Udall of New Mexico, Dennis C. Vacco of New York, Michael F. Easley of North Carolina, Heidi Heitkamp of North Dakota, Betty D. Montgomery of Ohio, W. A. Drew Edmondson of Oklahoma, Thomas W. Corbett, Jr., of Pennsylvania, Pedro R. Pierluisi of Puerto Rico, Jeffrey B. Pine of Rhode Island, Charles Molony Condon of South Carolina, Mark W. Barnett of South Dakota, Jan Graham of Utah, Jeffrey L. Amestoy of Vermont, Julio A. Brady of the Virgin Islands, James S. Gilmore III of Virginia, and William U. Hill of Wyoming; for the State of Wisconsin by James E. Doyle, Attorney General, and Sally L. Wellman and Mary E. Burke, Assistant Attorneys General; for the Menninger Foundation et al. by Philip Allen Lacovara, James C. Geoly, and Robert Teir; and for the Washington Legal Foundation et al. by Daniel J. Popeo and Richard A. Samp. Briefs of amici curiae urging affirmance were filed for the American Civil Liberties Union et al. by Scott A. W. Johnson, Laura J. Buckland, Steven R. Shapiro, Christopher A. Hansen, and Bruce Winick; for the American Psychiatric Association by Richard G. Taranto; for the National Association of Criminal Defense Lawyers et al. by David A. Reiser, Jennifer P. Lyman, Barbara E. Bergman, and James F. Vano; for the National Mental Health Association by Ira A. Burnim; for the SeattleKing County Defender Association et al. by Robert C. Boruchowitz, Addie Opinion of the Court JUSTICE THOMAS delivered the opinion of the Court. In 1994, Kansas enacted the Sexually Violent Predator Act, which establishes procedures for the civil commitment of persons who, due to a “mental abnormality” or a “personality disorder," are likely to engage in "predatory acts of sexual violence." Kan. Stat. Ann. §59-29a01 et seq. (1994). The State invoked the Act for the first time to commit Leroy Hendricks, an inmate who had a long history of sexually molesting children, and who was scheduled for release from prison shortly after the Act became law. Hendricks challenged his commitment on, inter alia, "substantive" due process, double jeopardy, and ex post facto grounds. The Kansas Supreme Court invalidated the Act, holding that its precommitment condition of a "mental abnormality" did not satisfy what the court perceived to be the "substantive" due process requirement that involuntary civil commitment must be predicated on a finding of "mental illness." In re Hendricks, 259 Kan. 246, 261, 912 P. 2d 129, 138 (1996). The State of Kansas petitioned for certiorari. Hendricks subsequently filed a cross-petition in which he reasserted his federal double jeopardy and ex post facto claims. We granted certiorari on both the petition and the cross-petition, 518 U. S. 1004 (1996), and now reverse the judgment below. I The Kansas Legislature enacted the Sexually Violent Predator Act (Act) in 1994 to grapple with the problem of managing repeat sexual offenders. Although Kansas al Hailstorks, John Stuart, Eric Janus, John T. Philipsborn, and Bernadette Foley; and for the Washington State Psychiatric Association by David A. Summers. David B. Robbins filed a brief for the Association for the Treatment of Sexual Abusers as amicus curiae. 1 Subsequent to Hendricks' commitment, the Kansas Legislature amended the Act in ways not relevant to this action. See, e. g., Kan. Stat. Opinion of the Court ready had a statute addressing the involuntary commitment of those defined as "mentally ill," the legislature determined that existing civil commitment procedures were inadequate to confront the risks presented by "sexually violent predators." In the Act's preamble, the legislature explained: "[A] small but extremely dangerous group of sexually violent predators exist who do not have a mental disease or defect that renders them appropriate for involuntary treatment pursuant to the [general involuntary civil commitment statute].... In contrast to persons appropriate for civil commitment under the [general involuntary civil commitment statute], sexually violent predators generally have anti-social personality features which are unamenable to existing mental illness treatment modalities and those features render them likely to engage in sexually violent behavior. The legislature further finds that sexually violent predators' likelihood of engaging in repeat acts of predatory sexual violence is high. The existing involuntary commitment procedure... is inadequate to address the risk these sexually violent predators pose to society. The legislature further finds that the prognosis for rehabilitating sexually violent predators in a prison setting is poor, the treatment needs of this population are very long term and the treatment modalities for this population are very different than the traditional treatment modalities for people appropriate for commitment under the [general involuntary civil commitment statute]." Kan. Stat. Ann. §59-29a01 (1994). As a result, the legislature found it necessary to establish "a civil commitment procedure for the long-term care and Ann. §59-29a03 (Supp. 1996) (changing notification period from 60 to 90 days); §59-29a04 (requiring state attorney general to initiate commitment proceedings). Opinion of the Court treatment of the sexually violent predator." Ibid. The Act defined a "sexually violent predator" as: "any person who has been convicted of or charged with a sexually violent offense and who suffers from a mental abnormality or personality disorder which makes the person likely to engage in the predatory acts of sexual violence." §59-29a02(a). A "mental abnormality" was defined, in turn, as a “congenital or acquired condition affecting the emotional or volitional capacity which predisposes the person to commit sexually violent offenses in a degree constituting such person a menace to the health and safety of others." §59-29a02(b). As originally structured, the Act's civil commitment procedures pertained to: (1) a presently confined person who, like Hendricks, "has been convicted of a sexually violent offense" and is scheduled for release; (2) a person who has been "charged with a sexually violent offense" but has been found incompetent to stand trial; (3) a person who has been found "not guilty by reason of insanity of a sexually violent of fense"; and (4) a person found "not guilty" of a sexually violent offense because of a mental disease or defect. 29a03(a), § 22-3221 (1995). §59 The initial version of the Act, as applied to a currently confined person such as Hendricks, was designed to initiate a specific series of procedures. The custodial agency was required to notify the local prosecutor 60 days before the anticipated release of a person who might have met the Act's criteria. §59-29a03. The prosecutor was then obligated, within 45 days, to decide whether to file a petition in state court seeking the person's involuntary commitment. §5929a04. If such a petition were filed, the court was to determine whether "probable cause" existed to support a finding that the person was a "sexually violent predator" and thus eligible for civil commitment. Upon such a determination, transfer of the individual to a secure facility for professional evaluation would occur. § 59-29a05. After that evaluation, |