This review describes the social identity approach to crowd behavior. Research based on the social identity approach to crowds has grown significantly in the last 20 years, both quantitatively and qualitatively. I organize the new research into three sections. First, I consider the recent findings on crowd density behaviors, heightened emotion in crowds, mass gatherings health, and crowd events that function to strengthen group identity. Second, I cover research on behavior in emergencies and discuss how models of crowd behavior have shaped policy and practice in emergency response. Third, I describe the recent research on psychological change in collective action, public order policing, and social influence. The increased number of practical applications demonstrates that the social identity research on the psychology of crowd behavior has value beyond the advances it has made in terms of theory.