The discussion notes the differences between serial murder and mass murder, explains the definition of mass murder, and describes the evolution of this crime over recent decades. The text explains that the typical mass murderer in the United States is a white male over age 25, often with no criminal record and no characteristics that attract notice. However, some mass murderers have longstanding behavioral histories indicating a predilection for violence; some have psychological disorders and a few are probably legally insane. Most mass murderers formulate their intentions over time and become obsessed about these intentions. In addition, most mass murderers use guns and kill their victims from a distance. The seven categories of motivation include perverted love, politics and hate, revenge, sexual homicide, premeditated execution or unplanned execution of witnesses, mental disorders, and unexplained. Tables describing individual murderers and their motivations, appended information about serial homicide and mass murder, index, and 41 references