Adolescence is a period that begins with puberty and ends with the transition to emerging adulthood.  For the purposes of this text and this chapter, we will define adolescence as the ages 12 to 18.  This chapter will outline changes that occur during adolescence in three domains: physical, cognitive, and social.  Within the social domain, changes in relationships with parents, peers, and romantic partners will be considered.  Next, the chapter turns to adolescents’ psychological and behavioral adjustment, including identity formation, aggression and antisocial behavior, anxiety, and depression, and academic achievement.  Finally, the chapter summarizes sources of diversity in adolescents’ experiences and development.