The paper examines gang-affiliated homicides (which involve gang members but do not result from gang activity), gang-motivated homicides (which result from gang activity) and nongang youth homicides in St. Louis. There were important differences as well as similarities in the time trends and event characteristics of the two types of gang homicide; in key respects the gang-affiliated homicides more closely resembled the nongang events. The gang-motivated events exhibited a somewhat distinctive spatial patterning, as might be expected from their connection to turf conflicts. However, all three homicide types were highly concentrated in racially isolated, disadvantaged neighborhoods, which remain the fundamental social facilitators of both gang and nongang violence. Results are consistent with so-called “threat” explanations of gang violence, which claim that gangs emerged as protective mechanisms in increasingly threatening urban environments during the 1980's. The results also offer powerful evidence of the clustering of both gang and nongang youth homicides in areas characterized by high levels of socioeconomic disadvantage and racial isolation. Notes, figures, tables, appendix, references