Identities are ubiquitous: they are embedded in individuals, expressed in interactions, and enacted in groups and organizations. We discuss this central aspect of the self, focusing on both the internal dynamics of identities (such as how to define and organize identities into different bases, and how identities operate within individuals) and the external dynamics (how identities relate to the social structure). While the internal dynamics address important foundational aspects of identities, the external dynamics show how identities relate to the micro, meso, and macro levels of social reality. We suggest two avenues of future research, each of which address the internal and external dynamics of identities, respectively.