Although it is well documented that intrapersonal and interpersonal risk factors are related tothe frequency of marijuana use, much less is known about the initiation of marijuana use. Thispaper reports on a longitudinal study of the personality, family, peer, and ecological predictors ofmarijuana onset. Survival analysis was applied to a sample of nonusers of illegal drugs, followedfrom age 9 years to the 20s. The major findings indicate that (a) youngsters who areunconventional are at a higher risk for marijuana initiation; (b) youngsters who associate withpeers who use marijuana or who smoke tobacco themselves are at increased risk for marijuanainitiation; (c) youngsters who identify with their parents are less likely to begin marijuana use;and (d) the predictors related to marijuana onset emerged during preadolescence, earlyadolescence, middle adolescence, late adolescence, and the 20s. Results are discussed within theframework of a family interactional perspective of development. Implications for prevention arediscussed.