Marijuana (Cannabis sativa) is the illicit drug most commonly used by teenagers in the United States. Although cannabis is not as addictive as other substances, such as heroin and cocaine, cannabis-dependent individuals still greatly outnumber those reporting dependence on other illicit drugs and the number of people seeking treatment for cannabis dependence continues to increase yearly. Despite these facts, there is a growing perception, particularly in adolescents and young adults, that cannabis is ‘harmless’ and there is currently much debate as to whether cannabis should be legalized. Unfortunately, most of the discussion and policies being made regarding cannabis have been done without significant consideration of scientific data. Our studies directly address the question regarding the long-term impact on the brain as a consequence of cannabis exposure during adolescence, a period of dynamic brain development.