Endothelial cells (ECs), which form the inner surface of the blood vessels, contact the blood, withstand mechanical pressure, and demonstrate heterogeneous reactions to exogenous and endogenous stimuli. ECs have unique properties in accordance with their niches and play an important role in regulating vascular homeostasis. Endothelial cells may undergo a dynamic phenotypic switch in terms of its heterogeneity, which may lead to endothelial dysfunction and a number of associated pathologies. Endothelial-mesenchymal transition (EndMT) is one of the possible molecular and cellular mechanisms of this kind. EndMT is characterized by phenotypic changes in ECs through which endothelial cells acquire new properties, i.e., start producing mesenchymal markers such as alpha-SMA and vimentin, change morphology, and become able to migrate. EndMT is a complex biological process that can be induced by inflammation, hypoxia, or oxidative stress and be involved in pathogenesis of cardiovascular disease. This review describes the key markers, inhibitors, and inducers of endothelial-mesenchymal transition and overall state-of-the-art of EndMT in cardiovascular diseases.