Microsporidia are parasitic fungi that can infect not only arthropods and fish, but also mammals including humans. This article describes the biology of these primitive fungi and gives an overview on recent developments in therapy. Encephalitozoonspp. and the species Enterocytozoon bieneusii are most prevalent in immunocompromised patients, causing systemic disease or diarrhea. Whereas Encephalitozoon infection can usually be effectively treated with albendazole, therapeutic approaches for E. bieneusii are still difficult.