This article is an attempt to clarify the role of the epithelial-mesenchymal transition (EMT) in normal endometrium, as well as in endometriosis. The significance of EMT and the reverse process of mesenchymal-epithelial transition in the restoration of the glandular component of the endometrium in the proliferative phase by means of glandular remodeling — the process of differentiation of mesenchymal stem cells of the endometrium to the epithelial cells of future uterine glands is described. It was also found that fibroblasts of the periglandular zone in the endometrium acquired an epithelial phenotype and contributed to the regeneration of the fallopian glands. In addition, the role of EMT in preventing endometrial fibrosis due to chronic functionally justified trauma is shown. EMT has a significant role in the survival of retrograde abandoned endometrial tissue, as well as in the formation of a focus of endometrioid heterotopia. Macrophages of peritoneal fluid play a significant role in promoting this process, since they recognize the retrograde abandoned endometrium as a «wound defect» and create favorable conditions for its survival in atypical anatomical and physiological conditions. For the formation of adenomyosis, EMT activation is also required. It is shown that by dedifferentiation of fibroblast-like cells of the basal layer of the endometrium and metaplasia of smooth myometrial myocytes, a border zone is formed in the area where there is no clear border between the endometrium and the myometrium, and the mobility of myofibroblasts causes the displacement of this zone deep into the myometrial tissue. The most significant condition for the above mentioned processes is the predominance of an anti-inflammatory subpopulation of macrophages (M2) with their influence on the endometrium. Based on the data of many researchers, we believe that this particular cell population is a key link in the pathogenesis of endometriosis, and the polarization of macrophages towards M2 was noted in the so-called leaky gut syndrome associated with chronic inflammatory bowel diseases and even dysbacteriosis. Identification of conditions that can lead to the prevalence of M2 and the formation of endometrioid heterotopias can become one of the most important achievements in understanding the etiopathogenesis of this pathology and become the basis for effective preventive work in relation to endometriosis — a global medical and social problem among the female population.