Microsporidia, unicellular parasites frequent in animals, were rarely reported in humans before the advent of AIDS. This immunodeficiency syndrome induces the emergence or resurgence of opportunistic infections such as microsporidioses. Since 1985, five species of microsporidia have been found in HIV-infected patients. One of these species was already known in animals whereas all others are new. An increasing number of cases of microsporidioses is detected due to the improvement of methods of diagnosis. According to a study conducted in USA, intestinal microsporidia appear to be the first cause of diarrhea in patients with AIDS. These parasites are intensively investigated as shown by the increasing number of studies published since 1993. Most data concern the diagnosis, pathology, therapy and epidemiology of human microsporidioses as well as the characterization of their agents. Experimental studies aiming to define the immune context of these infections are also reported.