Microenterprise approaches have been used in the context of both HIV prevention and treatment (e.g., adherence to antiretroviral therapy) with several highly vulnerable populations including women, female sex workers, and low-income HIV-positive individuals. This chapter defines and describes different approaches to microenterprise in the context of HIV prevention. The chapter also includes several case studies with vulnerable populations to evaluate the effectiveness of structural approaches to HIV prevention, including the IMAGE study, the Undarga study, the JEWEL study, and the Chennai study. The chapter concludes by outlining directions for future policy and research, including gaps and principles in research on HIV prevention.