This study considers the circumstances under which members of the Muslim American community voluntarily cooperate with police efforts to combat terrorism. Cooperation is defined to include both a general receptivity toward helping the police in antiterror work and the specific willingness to alert police to terror-related risks in a community. We compare two perspectives on why people cooperate with law enforcement, both developed with reference to general policing, in the context of antiterror policing and specifically among members of the Muslim American community. The first is instrumental. It suggests that people cooperate because they see tangible benefits that outweigh any costs. The second perspective is normative. It posits that people respond to their belief that police are a legitimate authority. On this view we link legitimacy to the fairness and procedural justice of police behavior. Data from a study involving interviews with Muslim Americans in New York City between March and June 2009 strongly support the normative model by finding that the procedural justice of police activities is the primary factor shaping legitimacy and cooperation with the police.