Is mass murder a historically new phenomenon that emerged in the 1960s? How has it changed over time? And what causes a person to commit multiple murders in a matter of hours or even minutes? This book explores these questions by examining 909 mass murders that took place in the United States between 1900 and 1999. By far the largest study on the topic to date, it begins with a look at the patterns and prevalence of mass murders by presenting rates from 1900-1999 and by describing the characteristics of mass killers. Placing the phenomenon within the broader social, political, and economic context of the twentieth century, the work examines the factors that have influenced trends in the prevalence of mass murder. It also discusses more than 100 case studies within three distinct periods of mass murder activity (1900-1939, 1940-1965, and 1966-1999) to illustrate more clearly the motives of mass murderers and the circumstances surrounding their crimes. The final chapters take a look at media coverage and the role it has played in the social construction of mass murder.