Analysis reveals that serial killers are typically white males in their late 20s or 30s who target strangers near their work or home. These killers tend to be sociopaths who satisfy personal needs by killing with physical force. Mass murders are demographically similar to serial killers. Mass murderers generally kill people they know well. They act deliberately and methodically to execute the victims in the most expedient way: with a gun. Both types of murderers are motivated by power, revenge, loyalty, profit, or terror. The research literature on multiple murder is more speculative than definitive and is based mainly on anecdotal evidence rather than hard data. Future studies should make greater use of comparison groups and seek life-cycle explanations that go beyond early childhood and that recognize the unique patterns and characteristics of multiple murderers. A research focus on these kinds of murder in the extreme may also aid the understanding of more commonplace forms of interpersonal violence. Tables and 89 references