This study examined the space-time patterns of HIV risk behaviors among injection drug users (IDUs) and HIV prevention services in the San Francisco Bay Area from 1987 to 2005. The project team modeled the relationship among neighborhood characteristics, neighborhood HIV prevention services, and individual HIV risk behavior outcomes among IDUs. Research on HIV and illicit substance use has traditionally focused solely on individual-level characteristics that confer risk. Emerging research in the areas of HIV and illicit substance use has identified the importance of considering social environmental factors, including access to health care services, when studying individual health behaviors. This study involved applying newly emergent spatial analytical methods to secondary data sets to refine the knowledge regarding how neighborhood characteristics might be associated with access to HIV prevention services and HIV risk among street-based IDUs. RTI's work adapted the well-known Behavioral Model of Vulnerable Populations (BMVP) to incorporate a well-developed conceptualization of the spatial interaction between contextual variables and individual behavior.