Evil is sometimes thought to be incomprehensible and abnormal, falling outside of familiar historical and human processes. And yet the twentieth century was replete with instances of cruelty on a massive scale, including systematic torture, murder, and enslavement of ordinary, innocent human beings. These overwhelming atrocities included genocide, totalitarianism, the Holocaust, and the Holodomor. This Element underlines the importance of careful, truthful historical investigation of the complicated realities of dark periods in human history; the importance of understanding these events in terms that give attention to the human experience of the people who were subject to them and those who perpetrated them; the question of whether the idea of 'evil' helps us to confront these periods honestly; and the possibility of improving our civilization's resilience in the face of the impulses towards cruelty to other human beings that have so often emerged.