Histomoniasis, caused by the protozoan, Histomonas meleagridis, is an economically important disease of turkeys, and it also affects several other species of domesticated and wild Galliformes, including chickens. Under natural conditions, the parasite is transmitted through eggs of a nematode, Heterakis gallinarum, that shares its hosts with Hi. meleagridis. The protozoan infects tissues of both male and female He. gallinarum and eventually is carried within the worm egg. Histomonas meleagridis more readily infects and develops in chickens, and the proximity of chicken farms is a major risk factor for outbreaks in turkeys. Chemoprophylaxis had controlled Hi. meleagridis in turkeys very successfully, but histomoniasis has recently reemerged in turkeys because anti-histomonal drugs are no longer permitted by the United States Food and Drug Administration because of the concerns for residual toxins in poultry meat. Horizontal transmission of the protozoan in the absence of worm eggs remains a mystery because the flagellate trophozoite excreted in the feces of turkeys is not viable for any length of time. A proposed resistant stage of the protozoan has not yet been conclusively demonstrated. Here we review the discovery of the protozoan and the current status of the disease and its control.