Since the beginning of the AlDS epidemic in 1981-1982, increasing numbers of cases of microsporidial infection in humans have been reported. Species of the genus Enterocytozoon, Encephalitozoon, Vittaforma, Pleistophora and Trachipleistophora have been implicated as being the causal agents of different diseases, both in immunocompromised hosts and in healthy individuals. The diagnostic methods employed are mainly based upon modified chromotrope staining, transmission electron microscopy (TEM), immunogenic analysis and DNA analysis. The mode of transmission to humans is uncertain, although some species have been found to infect animals, suggesting their zoonotic nature.