This chapter addresses Erik Homburger Erikson’s original concept of identity and describes how that concept evolved through the course of his writings. A number of attempts have been made to operationalize Erikson’s concept of identity for research purposes, and this chapter turns to one of the most popular of these efforts, the work of James Marcia. The empirical and clinical origins of Marcia’s identity status approach is described, and the rationale for and development of Marcia’s identity status measure (individual interviews scored with a comprehensive, theoretically consistent manual) is overviewed. The construct validation of the statuses and, thus, of selected elements of Erikson’s theoretical concept of identity are detailed. A review of key empirical relationships between identity status and selected personality variables, antecedent and consequent conditions, as well as developmental patterns of identity change over time is presented. The chapter then addresses more recent identity approaches that also assess exploration and commitment processes, along with their strengths and limitations. The clinical implications of Marcia’s identity statuses are discussed, and the chapter concludes with thoughts on the past, present, and future of identity studies.