This book examines identity theory’s centrality within social psychology and its foundations within structural symbolic interaction, highlighting its links not only to other prominent sociological subfields, but also to other theoretical perspectives within and beyond sociology. The book provides a synthetic overview outlining the intellectual lineage of identity theory within structural symbolic interactionism, and how the Indiana School of identity theory and research, associated especially with Sheldon Stryker, relates to other symbolic interactionist traditions within sociology. It also analyses the latest developments in response to the push to integrate identity theory, which initially focused on role identities, with the study of personal, group and social identities. Further, it discusses the relationship between identity theory and affect control theory, providing a sense of the many substantive topics within sociology beyond social psychology for which the study of identity has important, sometimes underappreciated implications. The book concludes with a chapter summarizing the interrelated lessons learned while also reflecting on remaining key questions and challenges for the future development of identity theory.