Kaminski seeks to explain murders of police in 190 cities and four time periods (1987, 1990, 1993, and 1996). His hypotheses are that differences in levels of officer exposure (e.g., arrests) and physical and social guardianship (e.g., mandatory vest-wear policies; proportion of one- vs. two-officer patrol units) influence opportunities for homicides after the effects of proximity to motivated offenders are accounted for. Social and physical guardianship are not significant. Homicides of police are primarily a function of exposure and proximity to motivated offenders. In addition, police departments employing more female officers and those located in the south experience more homicides. The results indicate that murders of police are determined by criminogenic structural conditions, arrest activity, and the number officers deployed in the field.