The non-taxonomic term microsporidia relates to a group of organisms belonging to the order Microsporida of the phylum Microspora. Microsporidia are obligate intracellular spore-forming protozoa and have no metabolically active stages outside the host cell. Their host range is extensive and includes most invertebrates and all 5 classes of vertebrates. More than 100 microsporidial genera and almost 1000 species have now been identified. 5 genera (Enterocytozoon, Encephalitozoon, Septata, Pleistophora and Nosema) and unclassified microsporidia have been associated with human disease. Only 10 cases of microsporidiosis have been described among persons not infected with HIV. In contrast, microsporidia have gained increasing attention as important opportunistic pathogens in the evolving pandemic of HIV infection. Diagnosis depends on morphological demonstration of the organisms themselves. The potential sources and modes of transmission of human microsporidial infections are uncertain. The clinical manifestations of microsporidiosis are diverse and include intestinal, pulmonary, ocular, muscular, and renal disease. Preliminary observations of the possible utility of albendazole for infections due to Septata intestinalis and Encephalitozoon sp. have been reported. The success of therapy for intestinal Enterocytozoon bieneusi infection has been limited. Cyclospora sp. are recently described protozoa capable of causing diarrhea in immunocompetent and immunodeficient patients. Groups at risk for infection are children in the developing world, travellers and HIV-infected patients. Diagnosis depends on light-microscopic detection of oocysts in stool smears stained with acid-fast stains. Diarrhea is usually self-limiting. Diarrhea, however, may often last weeks to months, causing significant morbidity. Cotrimoxazole appears to be the drug of choice for treatment of Cyclospora infection.