This article examines the impact of sociodemographic characteristics and psychosocial factors on the probability and frequency of marijuana use and, for youths initiating use, on their frequency of use over four time points. The sample consists of 278 justice-involved youths completing at least one of three follow-up interviews as part of a National Institute on Drug Abuse (NIDA)-funded study. A two-part growth model was estimated to examine the probability of engaging in marijuana use and change in marijuana frequency of use over time. In addition, covariates for intervention status, gender, age, race, ethnicity, psychological functioning, alcohol use, and self-reported delinquency were included as baseline predictors in the model. The results of our two-part latent growth model reveal (1) a latent tendency to engage in marijuana use and a conditional, continuing decision on frequency of marijuana use, and (2) different patterns of relationships among age, psychosocial variables, and the probability and frequency of marijuana use. Implications for analytical, policy, and intervention service needs are discussed. (Contains 4 tables, 1 figure, and 3 footnotes.)