This volume explores over 900 mass murders that took place in the U.S. between 1900-1999. Organized chronologically, it records mass murder rates within three distinct periods (1900-1939, 1940-1965, and 1966-1999) offering brief case studies of more than 100 mass murders chosen to reflect the dominant themes in such crimes for each period. In addition, cases are placed within a historical context by comparing trends in mass murder activity with previous research on homicide and serial murders. The work also explores factors that have influenced the prevalence of mass murder in the United States including characteristics of mass murderers and the circumstances surrounding their crimes.