This article explores differences in the social context in which crack sales and use and HIV risk take place in seven low-income communities in San Salvador, and structural factors that may influence these differences. The organization of drug selling varied among the communities on a number of dimensions including: whether drug sales were open or closed systems; the type of drug-selling site; and the participation of drug users in drug-distribution roles. Drug-use sites also varied according to whether crack was used in private, semiprivate, or public spaces, and whether individuals used drugs alone or with other drug users. Three patterns of drug use and selling were identified based on the dimensions outlined above. Structural factors that influenced these patterns included the geographic location of the communities, their physical layout, gang involvement in drug sales, and police surveillance. Implications for HIV risk and prevention are explored for each pattern.