In addition to alcohol, marijuana is one of the most commonly used substances of abuse among adolescents and young adults. Its effects on the brain and on behavior are poorly understood. Funded by the National Institute on Drug Abuse and NIAAA, we are examining the longitudinal impacts of heavy chronic marijuana use in college-aged individuals who began to use the drug when they were teenagers. Findings to date suggest that heavy marijuana use is associated with disruptions in working memory, planning skills, verbal learning and memory, and motivated decision-making. We also find evidence of disruption in the young adult development of frontal white matter tracts and in striatal glutamate activity.