In a changing public health landscape in which local, state, and federal agencies must confront threats of bioterrorism, emerging infections, and numerous chronic diseases, transmission of HIV among injection drug users (IDUs) continues to be an important public health issue and one of the driving forces behind the HIV epidemic. Using a computerized MEDLINE search of published articles from January 1981 through October 2005, we conducted a literature review of practical epidemiologic aspects of HIV/AIDS among IDUs in the United States. Although recent trends indicate a decline in the proportion of newly diagnosed HIV infections associated with injection drug use, drug-use behaviors overall still account for 32% of new HIV diagnoses. Factors in addition to syringe sharing contribute to HIV transmission among IDUs: risky sexual behaviors, sharing of drug preparation equipment and drug solutions, and contextual and social factors. Promising approaches for HIV prevention include rapid HIV testing, office-based substance abuse treatment, behavioral interventions, improved communication about syringe exchange programs, and case management. HIV among IDUs continues to be an important public health problem in the 21st century. It is imperative that public health agencies continue to monitor and combat the HIV epidemic among IDUs to ensure that hard-won gains will not be eroded.