Adolescence is a period of heightened emotionality and increased risk for mental illness, affecting as many as one in five persons. This article reviews recent human imaging and animal studies that demarcate adolescent specific changes in brain and behavior that may help to explain this period of increased risk for psychopathology. We highlight adolescence as a sensitive period when: 1) the environment has particularly strong influences on brain and behavior and 2) normative changes in brain development can lead to an imbalance between rapidly changing limbic circuitry and relatively slower developing prefrontal circuitry. This imbalance can be exacerbated by both genetic and environmental influences leading to less capacity to regulate emotions and higher risk for psychopathology. We discuss these findings in the context of understanding who may be at greatest risk for psychopathology and when and how to best treat symptoms of emotional dysregulation.