Migration is one of many social factors contributing to the spread of HIV and other blood-borne or sexually transmitted infections (STI). Bringing together large numbers of people from diverse settings, the process of migration moves infected individuals to diverse geographic locations. Injection drug users (IDU) are a relatively mobile group, often moving between cities, smaller communities, and across international borders for reasons of work, security, or access to narcotics. This mobility indicates the potential for IDU who engage in risky behavior outside their home areas to transmit HIV infection to other IDU, their sex partners, and others in the population. The objectives of this review are to examine: (1) the influence of drug trafficking and the spread of drug use on the diffusion of HIV, (2) the influence of migration on drug use and HIV-related risk behaviors among migrants, and (3) the mobility patterns of IDU and its role in the spread of HIV. We also discuss the potential policy implications of addressing prevention and care issues in mobile drug using populations.