This volume presents recent advances in identity theory, which is a prominent and active theory in sociological social psychology and a versatile framework for explaining the sources of identities, how they develop, how they operate in situations and groups, and how they influence behavior and well-being. The volume is organized around new theoretical developments, measurement techniques, and research in the field. Theoretical developments covered in the volume sharpen, reframe, and expand fundamental concepts in identity theory. State-of-the-art techniques for measuring identities assess, refine, and update existing measures. New research in the volume addresses both individual processes and outcomes and group processes and outcomes. The chapters together showcase the wide applicability of identity theory to a host of identities, such as the religious, gender, sexual, physical attractiveness, racial/ethnic, parent, student, partisan, and group member identities. The volume editors introduce identity theory and provide an overview of the chapters. In the last chapter, they describe how this volume points to future directions for advancing theory, measurement, and research in identity theory.