Since the beginning of the acquired immunodeficiency syndrome (AIDS) epidemic, malignancies have been an important feature of this disease. Several cancers, including Kaposi sarcoma (KS), certain aggressive B-cell lymphomas, and cervical cancer, are considered AIDS-defining when they occur in patients infected with human immunodeficiency virus. Most AIDS-defining tumors are associated with one of 3 DNA viruses: KS-associated herpesvirus, Epstein-Barr virus, or human papillomavirus. With the introduction of highly active antiretroviral therapy (HAART), the incidence of KS and certain lymphomas has decreased, whereas that of other tumors, such as cervical cancer, has undergone little change. Several new drugs and therapies have been developed for KS and AIDS-related lymphomas, and these treatments, plus the development of HAART, have contributed to improvements in morbidity and mortality. At the same time, the improved overall survival of patients with HAART has contributed to an increase in the number of patients living with AIDS in developed countries such as the United States. With the development of HAART and improved prevention and treatment of opportunistic infections, an increasing percentage of the deaths in AIDS patients have been from malignancies. Strategies for prevention, screening, and therapy remain important areas of research in this developing field.