People use marijuana for nonmedical reasons because they like how it affects the brain, and therefore their mind and subjective experience. However, regular use of marijuana alters brain structure and function in ways that persist well beyond the period of acute intoxication. Progressively more powerful research tools are documenting effects in high risk populations with considerably less than daily use. These impacts are generally reversible but may be permanent if they occur during stages of rapid neurodevelopment typical of early adolescence. The volume of gray matter and the number of synaptic connections can be reduced by marijuana use, most importantly in the hippocampus, amygdala, and frontal lobes. Gender differences exist in the impact of marijuana on cortical thickness during adolescence. The functional integrity of white matter interconnecting areas of the brain can also be reduced by chronic marijuana use, which interferes with the endogenous cannabinoid system’s formation of the microtubule skeleton within axons.