Currently, the prevalence of adenomyosis does not tend to decrease and it is being diagnosed more and more frequently in young women who have not performed their reproductive function, and therefore further study of the pathogenesis of adenomyosis is necessary, and the search for new non-invasive diagnostic methods and pathogenetically substantiated drug therapy are an extremely relevant tasks. This review presents the main theories of the development of adenomyosis. The features of the state of the endometrium and myometrium in patients with internal endometriosis were examined: the changes of the expression of various receptors and cytokines based on data from domestic and foreign literary sources of electronic databases such as PubMed, CyberLeninka, Google Scholar in the period from 2004 to 2020. Particular emphasis was given to the discussion of the research results of recent years. The review presents data on the expression of estrogen and progesterone receptors, cell adhesion molecules, immunological, genetic factors (natural killer cells (NK), osteopontin (OPN), leukemia-inhibiting factor (LIF), HOXA10 and HOXA11 genes) in the eutopic endometrium in adenomyosis, which plays a key role in the processes of implantation and gestation. Attention was drawn to the role of the epithelial-mesenchymal transition (EMF) in the pathogenesis of adenomyosis, and the main signaling pathways in the development of the disease were described. Further study of the molecular mechanisms responsible for the development of adenomyosis will reveal the disease at an early preclinical stage and will become the basis for the development of highly informative diagnostic methods and new directions of pathogenetic treatment.