The presence of ongoing enterprises, sustained by a purpose and life beyond those of the current participants, distinguishes drug- and gang-related homicides. Without these enterprises that are sustained over time within distinctive spatial areas, violence would be reduced to individual acts precipitated by unique features of interactions between distinct individuals. Consistent with expectation, drug and gang homicides display substantial concentrations of violent incidents spatially and temporally within different neighborhoods in a city. The authors also find evidence for localized temporal dependencies in which a recent homicide alters the probability of another homicide in the same local area. Both drug and gang homicides display evidence of self-limiting suppression effects that inhibit extended periods of sustained high rates of these types of violence in the same area. Finally, there is evidence of cross-type diffusion from drug and gang homicides to subsequently higher rates of other gun homicides involving young offenders or victims.