How do otherwise ordinary people become perpetrators of genocide? Why are groups targeted for mass killing? How do groups justify these terrible acts? While there are no easy answers to these questions, social psychologists are especially well positioned to contribute to our understanding of genocide and mass killing. With research targeting key questions -such as how negative impressions of outgroups develop and how social influence can lead people to violate their moral principles and other norms - social psychologists have much to teach us about why groups of people attempt to exterminate other groups, why people participate in such atrocious projects, and how they live with themselves afterwards. By bringing together research previously available only to readers of academic journals, this volume sheds crucial light on human behavior at the extremes and in doing so, helps us take one more step towards preventing future tragedies.