AIDS is an acquired immunodeficiency syndrome caused by the lentivirus HIV and characterized by a successive depletion of CD4 helper/inductor lymphocytes. The humoral and cellular immune deficiency is the basis for the development of opportunistic infections and tumors. Because of a multitude of different bacterial, viral, and parasitic opportunistic agents, AIDS is typified by a number of divergent clinical symptoms. As with many lentiviruses, HIV is difficult to demonstrate in the organism, especially in asymptomatic carriers. Although the rate of infection in peripheral lymphocytes appears to be low, there is an increasing amount of virological and immunological data which help to explain the slow but irreversible failure of the immune defense. We still know relatively little about the pathogenic mechanisms of HIV, although a number of the presently available experimental results provide useful starting points for subsequent investigations. Peter H. Duesberg recently published that HIV and AIDS may well be correlated, but stated that HIV is not the cause of AIDS. Duesberg bases his hypothesis on the fact that HIV fulfills neither Koch’s classic postulates nor several more of his own postulates for viral pathogenesis. Following the summary of individual pathogenic mechanisms of HIV infection, the separate points of Duesberg’s hypothesis are discussed in detail. It is made very clear that the magnitude of epidemiologic, clinical, and experimental observations and results argue for a causal role of HIV and A