HIV is the etiologic agent of AIDS. AIDS results from the loss of cells that are central to immune responses, T lymphocytes that express the CD4 protein on their surface. This paper relates HIV structure and replication to the clinical course of HIV infection. The virology of HIV replication is discussed first at the cellular and molecular levels. The course of HIV infection in vivo then is discussed and related to HIV replication. Finally, models that have been proposed to explain the mechanism whereby HIV causes immunodeficiency are considered. Although much is known about the growth of the virus both in vitro and in vivo, many questions remain about how HIV can deplete CD4-positive T lymphocytes and cause AIDS.