The BBC Prison Study was an experimental case study in which participants were randomly assigned to groups as prisoners or guards. This paper examines the impact of interventions designed to increase prisoners' sense of shared social identity on processes of leadership. It presents psychometric, behavioral, and observational data which support the propositions that (a) social identity makes leadership possible, (b) effective leadership facilitates the development of social identity, and (c) the long-term success and failure of leadership depends on the viability of identity-related projects. The study also points to the role of identity failure in precipitating change in general and the emergence of authoritarian leadership in particular. Findings provide integrated support for claims that social identity and self-categorization processes are fundamental to the leadership process and associated experiences of collective efficacy. (Contains 2 figures, 1 table, and 5 footnotes.)