This paper aims to extend the social identity approach to crowd behaviour (Reicher, 1984, 1987) in order to understand how crowd events, and crowd conflict in particular, develop over time. The analysis derives from a detailed account of a violent confrontation between students and police during a demonstration held in November 1988—the so-called ‘Battle of Westminster’. It focuses on how students came to be involved in the conflict, how the conflict spread and upon the psychological consequences of involvement. This analysis is used to develop general hypotheses concerning the initiation and development of collective conflict. It is concluded that, while the social identity model is of use in understanding these phenomena, it is necessary to recognize how social categories are constructed and reconstructed in the dynamics of intergroup interaction.