Microsporidiosis is an ubiquitous opportunistic disease that usually appears in immunocompromised patients: AIDS patients or organ-transplant recipients. The infectious agents of disease are fungi-related obligate intracellular parasites - microsporidia. Alongside with Cryptosporidium and Cytomegalovirus, these parasites are the most common causative agents of diarrhea in HIV-infected patients. Intestinalform of microsporidiosis has been mostfrequently observed, but microsporidia can affect almost any organs of the human body, eyes, lungs, muscles, organs of the nervous system. The present paper overviews the current data on the etiology, pathogenesis, epidemiology, clinical presentation, diagnosis and treatment methods of microsporidiosis.