More than 20 years of research in implementing interventions for IDUs indicate that HIV transmission among these users can be prevented, slowed, and stopped with the appropriate intervention. Intervening early with multiple strategies can prevent epidemic spread and its consequences. Even after prevalence has increased substantially, prevention interventions can reduce the further spread of HIV in drug-using populations and transmission into other populations. Although research results have been promising, it is unknown if self-reported behavior change is correlated with reducing rates of HIV transmission. This study will determine the efficacy of a peer-educator, network-oriented intervention to prevent HIV transmission among substance users and their risk network members. IDUs will be recruited using street and community outreach methods in Chiang Mai and Chiang Rai, Thailand and in Philadelphia, Pennsylvania, USA.