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Müllerianosis may be defined as an organoid structure of embryonic origin; a choristoma composed of müllerian rests - normal endometrium, normal endosalpinx, and normal endocervix - singly or in combination, incorporated within other normal organs during organogenesis. A choristoma is a mass of histologically normal tissue that is “not normally found in the organ or structure in which it is located” (Choristoma, 2006). Müllerian choristomas are a subset of non-müllerian choristomas found throughout the body. Histologically, endometrial-müllerianosis and endometriosis are both composed of endometrial glands and stroma, but there the similarity ends. Their pathogenesis is different. Sampson faced the same difficulty with pathogenesis and nomenclature when he wrote: “The nomenclature of misplaced endometrial or müllerian lesions is a difficult one to decide upon.” “The term müllerian would be inclusive and correct, but unfortunately it suggests an embryonic origin.” Sampson then divided “misplaced endometrial or müllerian tissue” into “four or possibly five groups, according to the manner in which this tissue reached its ectopic location” (Sampson, 1925). Sampson’s classification of heterotopic or misplaced endometrial tissue is based on pathogenesis: 1) “direct or primary endometriosis” [adenomyosis]; “a similar condition occurs in the wall of the tube from its invasion by the tubal mucosa” [endosalpingiosis]; 2) “peritoneal or implantation endometriosis;” 3) “transplantation endometriosis;” 4) “metastatic endometriosis;” and 5) “developmentally misplaced endometrial tissue. (I admit the possibility of such a condition, but have never been able to appreciate it.)” (Sampson, 1925). It is precisely this condition “developmentally misplaced endometrial tissue,” [müllerianosis] that is the subject of this review.