Hepatitis C virus (HCV) infection is a major cause of liver disease and hepatocellular carcinoma worldwide, as well as the leading cause of liver transplantations in the United States. As a result of similar modes of transmission, approximately 30% of HIV-infected individuals are co-infected with HCV. Among intravenous drug users, almost 90% of people infected with HIV are also infected with HCV. Because of treatment with highly active anti-retroviral therapy, HIV-infected individuals have improved survival and are no longer suffering from opportunistic infections and malignancy as in years past. As a result, co-infection with HCV has now become a frequent cause of morbidity and mortality in HIV-infected individuals. Furthermore, liver disease secondary to HCV infection is now the leading cause of hospital deaths in HIV-infected people in the US. HIV infection accelerates the course of HCV-related liver disease and viremia. It is less clear whether HCV infection affects the clinical course of HIV; however, HCV-related liver disease can limit many individuals from receiving anti-HIV therapy. HIV/ HCV co-infection is common, and serious. Physicians caring for HIV-infected patients worldwide must now address hepatitis C virus co-infection.