HYPOTHESES: (1) Participants in the motivational enhancement treatment check-ins (MCI) condition will reduce their use of marijuana more than participants in the computer based (Comp) condition and report fewer negative consequences. These differences are expected to be greater at later follow-ups (i.e, 9, 12, and 15 months) after repeated check-ins exert a cumulative effect. (2)Engagement in CBT sessions will be greater for those in the MCI condition and will partially mediate the effect on marijuana related outcome indices. (3) The MCI intervention will lead to increased self-efficacy, and decreased normative perceptions of marijuana use by other teens that will also partially mediate the effect of the intervention.