Subjects will be excluded if they 1) meet criteria for substance dependence (other than for nicotine) based on consensus between the Composite International Diagnostic Interview- Substance Abuse Model (CIDI-SAM) and clinical interview, 2) have a history of or currently have psychosis (not explained by drug use), or bipolar disorder that is currently unstable, 3) are currently receiving psychotherapy or some form of behavioral therapy, 4) have a chronic medical illness or condition that would keep them from fully participating in physical activity, 5) have been started on a psychotropic medication in the 30 days prior to study inclusion, or 6) are pregnant.