<p>Understanding oneself is a fascinating yet challenging process that entails dealing with identity questions. The answers to such questions can be revealed through the commitments one makes in various life domains (such as educational paths, career choices and interpersonal relationships) and the social groups with which one identifies. Dynamics of identity formation and consolidation are particularly salient in adolescence, a period marked by biological, cognitive and social changes that prompt individuals to question their sense of self. In this Review, I discuss theoretical models of how personal and social identity develop and change. Next, I describe how personal and social identity processes vary across life domains and individuals, and over time. I then examine the implications of identity processes for individual mental health and well-being, interpersonal relationships, and coping with transitions, as well as social and collective outcomes. Finally, I review intervention studies designed to foster identity development and propose an agenda for future basic and applied research aimed at understanding how identity can be supported and leveraged to promote both personal well-being and more inclusive and cohesive societies.</p>