Neuroscience evidence played a dominant role in Roper (2005) and Graham (2010). In reaching its decisions in these cases, the US Supreme Court relied in large part on neuroscientific studies that suggested that there is clear evidence that significant changes in brain structure and function occur during adolescence, with the most important changes occurring in the prefrontal cortex and in the connections between the prefrontal cortex and other brain structures. This chapter provides a detailed description of the neurodevelopment, anatomy, organization, and changes in the reward-processing system of the adolescent brain, as well a discussion of adolescents’ inability to (a) weigh risks and rewards, (b) control impulses, and (c) resist peer influences.