Diarrhea is a major complication of HIV infection and adversely impacts health care costs, quality of life, and even survival of patients. There is a wide variety of potential causes of diarrhea in HIV-infected patients, and the number of pathogens found continues to increase with time. In addition, there is some controversy concerning the role of some organisms in the pathogenesis of diarrhea and the appropriate diagnostic evaluation of affected patients. This article reviews our current understanding of these pathogens and some of the diagnostic and therapeutic approaches for diarrhea associated with HIV infection.