Blastocystis sp. is an anaerobic unicellular micro-organism belonging to the kingdom Chromista, frequently found in the digestive tracts of humans and animals, the pathogenic role of which continues being controversial for human beings. Its genetic classification, which shows nine sub-types, some of which seem to have a role in cases with gastrointestinal symptomathology, opens a new field for research. In this article, an extensive revision is carried out which includes the historic development of the parasite, its taxonomy, epidemiology, morphology, vital cycle, as well as biochemical, cytochemical and genetic aspects, the pathogenic role in contrast with different variables which include the sub-types, quantity, response to the treatment and association with other pathogens. Finally, the clinical and therapeutic aspects are also reviewed.