Encouraged by the federal government, every state in the country has some version of the law, which gives the public easy access to information on sex offenders when they are released into the community.
But Wednesday was the first time the high court had dealt with its constitutionality.
The laws are named for 7-year-old Megan Kanka, who was killed in New Jersey in 1994 by a neighbor who had been convicted by a prior sex offense.
The criminal history of the sex offender was not generally known in the Kanka neighborhood.
The Supreme Court heard the challenges to the state laws in separate cases on Nov. 13. In both challenges, lower courts had struck down the individual state laws as unconstitutional.
In each case, the state and the Bush administration asked the Supreme Court to overturn the lower court and uphold the sex offender registries as lawful regulation rather than unconstitutional punishment.
The issue in the Alaska case was whether sex offenders convicted before the state enacted its version of the law in 1994 were being punished unconstitutionally after the fact.
The Constitution bans "ex post facto" laws that are passed after a crime was committed but add punishment for the crime.
Speaking for a 6-3 majority Wednesday, Justice Anthony Kennedy said the Alaska law passed constitutional muster.
In order for it to unconstitutionally require retroactive punishment, Kennedy said, the law would have to be punitive. However, since the law is a non-punitive civil regulation, its retroactive application does not violate the ex post facto clause.
The Connecticut case had a much broader sweep. The issue in Connecticut was whether being included in a publicly accessed sex offender registry -- without a new hearing proving that the offender remains dangerous -- violates the Constitution's guarantee of "due process," or a fair and rational procedure.
In the prevailing opinion, Chief Justice William Rehnquist said the due process clause does not entitle a defendant to a hearing to prove a fact -- future dangerousness -- that is not required by the law.
In other words, prior sex offenders must register in Connecticut, regardless of whether they are a future danger to the community.
The court was unanimous in the judgment, though three filed concurring opinions.
In the Alaska case, two men identified as John Doe I and John Doe II pleaded no contest to charges of sexually abusing minors. Both were released from prison in 1990.
Alaska enacted the Sex Offender Registration Act, its version of Megan's law, in 1994. It requires those who were convicted of sex offenses after 1984 to register with the state, providing their names, addresses, employers and vehicles. They must also be fingerprinted and photographed by police. Like Connecticut, most of that information is then provided to the public on an Internet sex offender registry.
The Does filed suit, contending the law violates the Constitution because it punishes them for conduct that occurred before the law was passed. A federal judge disagreed, but a federal appeals panel reversed, saying the effects of the law were punitive.
Alaska then asked the Supreme Court for review. The justices reversed the lower court Wednesday.
In the second case, Connecticut's version of the law requires the registration of anyone convicted of a crime in four categories: criminal offenses against a minor victim; non-violent sexual offenses; violent sexual offenses, and felonies committed for a sexual purpose.
Persons convicted of violent sexual offenses are registered for life; the rest are registered for 10 years.
Everyone who must be registered is required to provide the state Department of Public Safety with his or her name, fingerprints and other identifying factors, a photograph, criminal history record, address and a blood sample for DNA purposes.
Convicted violent sexual offenders also must supply information on any mental health treatment, and verify an address every 90 days.
All those being registered have to notify the state of a new address within five days. They must also tell the registering agency when they temporarily reside in another state, and must submit to having a new photograph taken any time the state requests it.
The state Department of Public Safety makes the registry available to the public, primarily through its "Sex Offender Registry" on the Internet. The registry can be reached through the Web pages of several television stations as well as the state's Web page at state.ct.us.
The database is searchable through the use of zip codes so members of the public can find out which convicted sex offenders are living in their neighborhoods.
Sometime before May 1999, the state added a statement to the registry that said the DPS has "not considered or assessed the specific risk of reoffense ... and has made no determination that any individual included in the registry is currently dangerous."
One registry enrollee, identified in the case only as John Doe, challenged the law in February 1999, saying it violated the "due process" guarantee of the 14th Amendment because it deprives him of a "liberty interest" -- his reputation and status under state law -- without a meaningful hearing.
When a federal appeals court eventually ruled in his favor, Connecticut Attorney General Richard Blumenthal asked the Supreme Court for review of the Doe case, which has been combined with a similar case filed by "Samuel Poe."
Wednesday, the Supreme Court reversed the lower court in the Connecticut case.
(Nos. 01-729, Smith and Botelho vs. John Doe I et al; and 01-1231, Conn. DPS et al vs. John Doe et al)