Crimes committed by juveniles are typically defined by legal and nonlegal definitions. Legal definitions tend to reflect State and Federal statutes on juvenile offenses and offenders; nonlegal definitions are derived from interpretations of the law, community norms, social trends, and delinquency authorities. Most legal or nonlegal definitions differentiate serious and violent juvenile offenders from juveniles who commit status offenses such as running away or truancy, which are applicable only to minors. The three primary sources for measuring juvenile criminality are official statistics recorded by police, victimization surveys, and self-report surveys. Official arrest statistics indicate that juvenile arrestees under age 18 accounted for 27.8 percent of total arrests for personal crimes in 1999, including rape, assaultive offenses, robbery, and theft. Crime victimization surveys are an important complement to arrest statistics in measuring juvenile crime and delinquency. These surveys contact randomly selected households and ask respondents about criminal victimization details from the perspective of the victim. This information includes estimations of the age of the perpetrators of such crimes. According to the National Crime Victimization Survey of 1995, nearly half the crimes of violence with multiple offenders were perpetrated by offenders who were age 20 and younger. For rape and sexual assaults, almost 56 percent were believed by victims to have been committed by multiple offenders younger than 20 years old. One in three single-offender victimizations were perpetrated by offenders under the age of 20. Self-report surveys ask juveniles themselves whether they have ever committed delinquent or criminal acts. Self-report data have shown that juveniles' involvement in crime and delinquency is far more widespread than arrest statistics alone would indicate. 7 tables and 3 figures