“Monsters exist, but they are too few in number to be truly dangerous.” Thus begins Guenter Lewy’s latest book, Perpetrators: The World of the Holocaust Killers (Oxford University Press, 2017), a welcome attempt to challenge the idea that all Nazi perpetrators were the same, and that they were all driven by the same bass motivations. Largely a synthesis of material previously only available in German, Lewy presents a typology of perpetrator types and dispels the idea that it was impossible for killers to walk away. He also presents arguably the most accessible analysis of the post-war justice available in English. Undoubtedly a must-read for anyone wishing to understand how and why people participate in acts of mass violence. Darren O’Byrne is a PhD student in History at Cambridge University. His dissertation, Political Civil Servants and the German Administration under Nazism, explores the dynamics of Civil Service behaviour under National Socialism, asking why senior administrators assisted the regime in pursuit of its ideological goals. He has forthcoming publications with the Journal of Contemporary History and the Routledge Studies in Genocide and Crimes against Humanity. He can be contacted at email@example.com or on twitter at @darrenobyrne1.