Hate is without question one of the most important problems of the modern world. It plays a significant role in wars between nations, ethnic and religious groups, as well as terrorism, genocide, and the like. To date, however, very little attention has been given by psychologists to understanding how hate evolves and is maintained, and which processes play a role in any kind of assault where hate is involved. The book at hand provides an overview of the psychological theories that deal with the subject of hate. None of these theories has yet been subject to extensive empirical testing and research. It then focuses on what may be the most comprehensive of the available theories of hate, the Duplex Theory of Hate (Sternberg, 2003). A Triangular Hate Scale to assess feelings of hate has been devised to provide a means to test the theory empirically and to draw conclusions about its validity; and three studies have been conducted both in the United States and in Germany. The goal is to provide an initial basis for further empirical research on the topic of hate, so that, eventually, psychological findings from this area can be applied to help people resolve the conflicts that currently dominate the lives of so many.