In many cases of mass violence and genocide there is ambiguity and uncertainty as to whether and how external bystanders (i.e., third parties) should respond. How does the way we construe genocide influence our evaluations of particular cases of mass violence and our willingness to intervene? In five studies, using content analyses and experiments, prototype theory is applied to this important social issue. Studies 1 and 2 examine the prototype structure of genocide; finding among a student and a community sample that some features are perceived as more central to genocide than others. Studies 3 and 4 show the effects of this prototype on the cognitive processing of the category. Study 5 investigates how this prototype structure affects evaluations of mass violence and support for political and military intervention. Taken together, these studies suggest that socially shared prototypes of genocide matter: The more a case of mass violence is represented in accordance with this prototype, the more people remember and respond to it, for example, by supporting policies aimed at preventing and halting mass violence. These findings have important policy implications for how cases of mass violence are framed and discussed in the public and political sphere.