The circumstances surrounding "justifiable homicides" by police are the focus of this data collection, which examines occurrences in 57 United States cities during the period 1970-1979. Homicides by on- and off-duty police officers serving communities of 250,000 or more were studied. Data were collected through a survey questionnaire sent to police executives of the 57 cities. The Federal Bureau of Investigation supplied data on justifiable homicides by police, including age, sex, and race data. The variables include number of sworn officers, number of supervisory officers, average years of education, department regulations about issues such as off-duty employment, uniforms, carrying firearms, and disciplinary actions, in-service training, pre-service training, firearms practice, assignments without firearms, on-duty deaths, and off-duty deaths. The study was funded by a grant from the National Institute of Justice to the International Association of Chiefs of Police.