In this chapter, I consider the roles of psychology and morality in genocide and in intense violence and mass killing. The starting point for both can be either difficult social conditions in a society or conflict between groups. But psychology and individual-level morality have central roles. Even though there are many societal, cultural, and institutional forces at work, the proximal influences leading to genocide or mass killing are psychological. As the participants undergo a grim evolution, progressing along a "continuum of destruction" (Staub, 1989), moved by psychological and social forces, moral principles and orientations can be subverted. As people respond to these forces, they may engage in profoundly immoral actions without even struggling with the immorality of their thoughts, feelings, and actions. How psychological forces can overwhelm or subvert moral principles and emotions is the primary focus of this chapter. I consider the psychology and morality of perpetrators, passive bystanders, and active bystanders who endanger themselves as they attempt to save lives.I define morality as principles, values, emotional orientations, and practices that maintain or promote human welfare (see also Chapters 13 and 14, this volume, for similar definitions). With the increasing awareness of our