Approximately 20 percent of the youth in the cohort reported belonging to a gang at the age of 13; by age 17, only 5 percent said they were gang members. Those who joined "hard core" gangs--defined as having a well-developed organization, symbols of gang membership, and a persistent deviant lifestyle--tended to remain in these gangs over these years, however. Gang membership was more common among teens from less affluent families without both parents living in the home. Gang membership was higher among youth who lived in deprived neighborhoods, suggesting that the socioeconomic status of their neighborhoods had a stronger link to gang membership than family characteristics. Girls were just as likely as boys to be gang members at age 13, but their gang membership declined more rapidly than that of boys in subsequent years. Rates of delinquency and substance use were much higher among gang members than nongang members between the ages of 13 to 17; this was true for girls as well as boys. The same youths committed more offenses while they were gang members than after they left the gang, which suggests that the peer pressure of gang membership is more important than individual characteristics in influencing delinquent behavior. This study drew on findings from the Edinburgh Study of Youth Transitions and Crime, a longitudinal research program that is exploring pathways into and out of offending for a cohort of approximately 4,300 youth who began secondary school in Edinburgh in 1998. The study uses self-report questionnaires, semistructured interviews, children's hearings records, teacher questionnaires, police and criminal statistics, a parent survey, and a geographic information system. 11 tables, 2 figures, and 26 references