Identity is the individual's psychological relationship to particular social category systems. This chapter summarizes how people create and negotiate their gender, racial, ethnic, sexual, and class identities. Theories, methods, and priorities in each of these content areas differ. However, each systematically excludes particular research participants and thus ignores the complexity of people's multiple social identities. Research suggests that gender, racial, ethnic, sexual, and class identities are fluid, multidimensional, personalized social constructions that reflect the individual's current context and sociohistorical cohort. However, far too little empirical work captures the richly textured, theoretical conceptions of identity development, maintenance, and change. Innovative methods for assessing the content and structure of people's identities now exist. Future research should include groups other than young children or college students; should explore functions of identity other than just self-esteem, adjustment, or well-being; and should implement methodologies that are longitudinal and that assess people's many social identities.