Why do countries adopt criminal legislation making it possible to prosecute government and military officials for human rights violations? Over the past thirty years, dozens of countries have prosecuted their own or other states’ officials for past atrocities. Criminalizing Atrocity tells the story of the global spread of national criminal laws against atrocity crimes—genocide, war crimes, and crimes against humanity—laws that have helped pave the way for this remarkable trend toward greater accountability. It traces the early-twentieth-century origins of national atrocity laws to a group of influential European criminal law scholars and explains the global patterns by which they have since spread. The book shows that understanding why countries criminalize atrocities requires understanding how they do so. In many cases, criminalization has not been the result of concerted government initiative, but of inconspicuous choices made by technocratic legal experts who have been delegated authority to draft large-scale reforms to countries’ criminal codes. Drawing on research in comparative law and norm diffusion, Criminalizing Atrocity explains how such reform projects prompt technocratic drafters to select legal ideas, like atrocity laws, that have been endorsed by their professional communities and deemed by drafters to be important features of a “modern” criminal code. To test this argument, Criminalizing Atrocity draws on a range of original quantitative and qualitative data, including in-depth case studies of Guatemala, Colombia, Poland, and the Maldvies, and a new, comprehensive dataset tracking the global spread of atrocity laws since Word War II. The book’s findings highlight the importance of professional communities in the modern renaissance of atrocity justice and the domestication of international legal norms.