The social identity approach (comprising social identity theory and self-categorization theory) is a highly influential theory of group processes and intergroup relations, having redefined how we think about numerous group-mediated phenomena. Since its emergence in the early 1970s, the social identity approach has been elaborated, re-interpreted, and occasionally misinterpreted. The goal of this paper is to provide a critical, historical review of how thinking and research within the social identity approach has evolved. The core principles of the theories are reviewed and discussed, and their effect on the field assessed. Strengths and limitations of the approach are discussed, with an eye to future developments.