Although the International Commission on Intervention and State Sovereignty identified the responsibility to prevent as the single most important aspect of its report The Responsibility to Protect, most scholarly and political attention has been given to the concept's reaction component rather than to its prevention component. This article aims to correct this imbalance by examining progress with, changes to, and attitudes toward the responsibility to prevent since the publication of the commission's report in 2001. It seeks to explain the relative neglect of prevention in debates about The Responsibility to Protect, arguing that the answer can be found in a combination of doubts about how wide the definition of prevention should be, political concerns raised by the use of prevention in the war on terrorism, and practical concerns about the appropriate institutional locus for responsibility. The article moves on to identify some basic principles that might help advance the responsibility to prevent.