Dehumanization has traditionally been understood as an extreme form of prejudice that enables violence and cruelty. Recent work indicates that it can also take subtle and everyday forms. We give an overview of early contributions to the psychological study of dehumanization before reviewing recent research on the attribution and denial of humanness. We emphasize our own research program, which proceeds from a theoretical analysis of two distinct senses of humanness, which when denied to others yield two forms of dehumanization. In one form, people are denied uniquely human attributes and likened to animals, and in the other, they are denied essentially human attributes and likened to machines. We discuss the ways in which these two forms of dehumanization are manifest in perceptions of individuals and groups, and speculate on the cognitive and motivational processes involved.