Although advances in neuroimaging have revealed much about the impact of substances on the brain, many questions remain concerning how substance use and other experiences during adolescence influence brain development, and how they promote or interfere with later health outcomes such as addiction and mental illnesses. To what extent do tobacco, alcohol, marijuana and other drug exposures alter neurodevelopmental trajectories; and how does that, in turn, affect academic achievement, social and emotional development, and other aspects of life? Conversely, to what extent do different neurodevelopmental trajectories put children at greater risk for substance use? How do different types of substance use interact, and can the effects of individual drugs be disentangled in individuals who use more than one? Are the impacts of substance exposures persistent, or do they reverse when someone stops using them? To find answers to these questions, we must also understand something more basic: What is the normal trajectory of healthy human neurodevelopment—and is there even a single trajectory?
