Mass shootings are arguably one of the worst manifestations of gun violence. As discussed in this book, media outlets, gun control and rights advocates, statute, law enforcement agencies, and researchers often adopt different definitions of mass killing, mass murder, and mass shooting, contributing to a welter of claims about the prevalence and deadliness of mass shootings. For the purposes of this book, mass shooting is defined as a multiple homicide incident in which four or more victims are murdered with firearms, within one event, and in one or more locations in close proximity. Chapter One of this book analyses mass shootings for a 15-year period (1999-2013). Chapter Two examines the United States use of two national data collection systems to track detailed information on homicides: the Federal Bureau of Investigations Supplementary Homicide Reports and the Centers for Disease Control and Preventions Fatal Injury Reports. Chapter Three presents data on homicide trends from 1992 to 2011. It describes homicide patterns and trends by age, sex, and race of the victim. It explores weapon use, with a focus on trends in firearm use and homicide trends by city size. It also includes special discussions of missing offender data and firearm use in nonfatal violent victimisations. Chapter Four provides comparative data from as far back as 1986 for context and analyses of firearms manufacturing trends over the years, and a broad picture of the state of firearms commerce in the United States today. It includes a state-by-state breakdown of the federal firearms licensees and a state-by-state breakdown of the National Firearm Act tax revenue information available through 2014.