Adolescence is a developmental period which is often characterized as a time of impulsive and risky choices leading to increased incidence of unintentional injuries and violence, alcohol and drug abuse, unintended pregnancy, and sexually transmitted diseases. Traditional neurobiological and cognitive explanations for such suboptimal decisions have failed to account for nonlinear changes in behaviour observed during adolescence, relative to childhood and adulthood. This chapter provides a biologically plausible conceptualization of the neural mechanisms underlying these nonlinear changes in behaviour, of a heightened sensitivity to incentives while impulse control is still relatively immature during this period. Recent human imaging and animal studies provide a biological basis for this view, suggesting differential development of limbic reward systems relative to top-down control systems during adolescence, relative to childhood and adulthood. Finally, a mathematical model is provided to further distinguish these constructs of impulsivity and risky choices to further characterize developmental and individual differences in suboptimal decisions during this period.