A mass murder is defined as the killing of three or more people at one time and in one location. The serial murder is the killing of three or more people in more than a 30-day period with a significant cooling-off period between the murders. A spree murder is the killing of three or more people usually within a 30-day period and typically during the course of another felony (such as a robbery). In the 1990's there was more than 100 cases of mass murder. There appears to be no reliable source regarding accurate data on the incidence of mass murder. Despite not knowing the exact number of people who fall victim to mass killers, it is usually found that a mass killer murders intimates as well as strangers. There have been cases of mass murder since early in U.S. history. Many of these acts of multicide are unrecorded. No single factor accounts for the personality of the mass killer. Most likely, the motivation to kill comes from outside the personality. The reasons for an act of mass murder are psychological gain and material gain. The killer is found to be geographically stable. The major types of mass killers are pseudocommando, the disciple, the family annihilator, the religious/ideological killer, the disgruntled citizen, the disgruntled employee, the set-and-run killer, and the psychotic mass killer. School shootings by youthful killers have occurred numerous times in this country’s history. Some of the common traits that these types of killers share are white male, student, rural or suburban settings, middle class background, disenfranchised, interest in internet and computer games, and interest in weaponry. Departments that investigate a mass murder case encounter problems that are not ordinarily present in many cases, such as turf protection, determination of the type of mass murder, and identification of the victims. References, index