According to social identity theory people derive part of their identity—their social identity—from the groups to which they belong (e.g., an identity as “student”, “woman”, “left-hander”, or “Barcelona supporter”). Social identities differ in strength and content. The strength component is conceptualized in terms of social identification (e.g., “I identity strongly with Europeans”), while the content of social identity is determined by the group’s features (e.g., colors associated with a soccer team) and norms (e.g., “real men don’t cry”). Social identity determines emotions (e.g., depression after a team loss) and behavior (e.g., discrimination against out-groups, or effort on behalf of one’s in-group). In this chapter we outline the basic features of social identity theory—from social categorization to coping with a negative social identity—and then discuss two important domains of application: health and organizations. We conclude by describing a social identity based intervention for improving inter-group relations in an educational setting.