in the United States, a law requiring that law-enforcement officials notify local schools, day-care centres, and residents of the presence of convicted sex offenders in their communities. The law is named after Megan Kanka, a seven-year-old New Jersey girl who was brutally raped and murdered in 1994 by a twice-convicted sexual offender who had been living across the street from her home. In various forms such statutes were passed by many U.S. states in the mid-1990s. In 1996 it was adopted as a federal law in the United States.

Concern about sexually related child abuse, and especially pedophilia, became intense during the 1980s, chiefly in the United States but also in Europe and elsewhere. Although the main threat to children continued to come from members of the victim's family, by about 1990 attention had come to be focused on sexual molestation committed by non-family members and strangers, and it was widely believed that offenders in such cases typically attacked many children before they were apprehended. In response, many U.S. states passed stringent sexual-predator laws (sexual-predator law). Under a law passed in Washington state in 1990, for example, convicted sex offenders whom prosecutors considered still dangerous were required to register with local police when they changed residence.

Following the attack on Megan Kanka, New Jersey enacted a law that created a three-tiered classification of offenders based on prosecutors' assessments of how likely it was that the offender would repeat his crime, and it required that police notify local residents of the presence of high-risk offenders. The law was soon imitated in other U.S. states and in Europe.

One unintended side effect of these laws has been the increase in the psychological trauma to which some victims were exposed. Traditionally, prosecutors attempted to arrange plea bargains with defendants in child-abuse cases in order to spare the child the ordeal of testifying in court. Some versions of Megan's law, however, require all persons convicted of a sexual offense against a minor, and not just those considered to be high-risk, to register with police. Thus, in the hope of avoiding the necessity of registration, which normally has devastating effects on the defendant's employment prospects, some defendants choose to go to trial rather than plead guilty. Megan's law has been criticized on other grounds by civil libertarians, who argue that the stigmatization and ostracism that inevitably result from registration constitute a second punishment for the same crime and make rehabilitation of the offender more difficult.