Adolescents engage in risk behaviors at an alarming rate, and particularly when they are with peers. Despite efforts to develop prevention and intervention programs, rates of risk-taking among adolescents is still relatively high. Adolescents continue to engage in physical fights and aggressive behavior, use substances such as alcohol and illicit drugs, and make decisions that impact driving and motor vehicle incidents. The regulation of emotions plays a significant role in adolescents’ decisions to engage in such risk behaviors.Examining adolescents’ emotion expression and regulation is therefore critical to identifying ways to support positive development. This dissertation project explored important regulatory mechanisms that underlie adolescents’ behavior in 108 adolescents, by examining synchrony between emotion expression and physiological arousal (change from baseline heart rate to tasking heart rate)during a risk-taking task. The study also assessed the extent to which the social context of peers shifts emotion expression and physiological arousal, and whether this is associated with adolescents’ behavioral and social outcomes, and if these associations vary by gender. Study results suggest that the presence of a peer influences adolescents’ emotion expression. Specifically, adolescents showed greater expression of emotion when completing a risk-taking task in the presence of a peer, than when they completed the task alone. Additionally, adolescent girls are generally more expressive than their male counterparts and equally expressive alone and with a peer, but adolescent boys express more when they are with a peer than when they are alone. Synchrony between emotion expression and physiological arousal was not evident, however results of supplemental analyses suggest that physiological arousal (change from baseline heart rate to heart rate during the task)plays a moderating role in the association between emotion expression and social competence. Findings from the proposed study may inform intervention and policy efforts to understand and promote positive development among adolescents. In particular, results may shift how adults understand and respond to adolescent behavior in social contexts such as classrooms.