Adolescence is a period of growing focus on social interactions and relationships. The peer context is one of the most significant developmental contexts in this transitional period and positive peer experiences contribute positively to adolescent well-being. Although negative peer influence on antisocial behaviour has received much attention, we know less about the positive power of peers such as on prosocial behaviour. Functioning in a social context involves affective and regulatory processes, as well as understanding others and mentalizing. In this article, I highlight neuroscientific studies examining prosocial decision-making in the peer context. Based on studies combining economic exchange paradigms with sociometric assessments of peer relations, I show that prosocial behaviour for friends involves activation of the ventral striatum and the temporoparietal junction, brain regions associated with affective-motivational processes and other-oriented thinking. Differential developmental trajectories of these brain regions across adolescence can be critical in understanding the developmental significance of peer relationships and its links with developmental outcomes.