**1. Introduction**

*Dientamoeba fragilis* is an enteric protozoan parasite that remains obscure and neglected. While many infections remain asymptomatic, it is now generally accepted that *D. fragilis* is account‐

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able for an active infection, concomitant with abdominal symptoms, nausea, and diarrhea. Moreover, controversy exists over the protective role of the parasite in priming the immune system in a beneficial way such as in selecting beneficial bacteria, keeping potential harmful microbial intruders at bay or producing metabolites beneficial to the host. Furthermore, the parasite's transmission mode remains a mystery. The microscopic identification and diagnoses of *D. fragilis*in stool requires skill and expertise; consequently, it is likely that many infections may go unidentified. Numerous studies have reported the effectiveness of treatment regimens using compounds such as paromomycin, hydroxyquinolines, and 5-nitroimidazoles, includ‐ ing metronidazole and tinidazole in the parasite eradication and the resolution of clinical symptoms. In addition, there is very little *in vitro* susceptibility data available for this parasite, making some current treatment options questionable. This chapter reviews the scientific literature relating to Dientamoeba's life cycle, prevalence, diagnosis, and pathogenicity.
