Erik Erikson proposed eight stages of psychosocial development across the life span.
Each life stage has its own psychosocial task, with the chief task of adolescence being solidifying one's sense of self, one's identity. This often means trying out a number of different roles. Social identity is the part of the self-concept that comes from a person's group memberships.
Erikson believed we need to achieve the following challenges: trust, autonomy, initiative, competency, identity (in adolescence), intimacy (in young adulthood), generativity, and integrity.