Although it is well documented that intrapersonal and interpersonal risk factors are related to the frequency of marijuana use, much less is known about the initiation of marijuana use. This paper reports on a longitudinal study of the personality, family, peer, and ecological predictors of marijuana onset. Survival analysis was applied to a sample of nonusers of illegal drugs, followed from age 9 years to the 20s. The major findings indicate that (a) youngsters who are unconventional are at a higher risk for marijuana initiation; (b) youngsters who associate with peers who use marijuana or who smoke tobacco themselves are at increased risk for marijuana initiation; (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, early adolescence, middle adolescence, late adolescence, and the 20s. Results are discussed within the framework of a family interactional perspective of development. Implications for prevention are discussed.