Endothelium is a thin layer of cells that coats the inner surface of the cardiovascular system and provides a link between the circulating blood and the vessel wall. Due to variety of functions and wide distribution in the body, endothelium can be thought of as an organ by itself. Endothelial dysfunction (ED) is a term which describes the state of damaged endothelium with related loss of function. ED consists of a wide range of pathophysiologic states, in span of localised intimal damage to global, persistent, inadequate endothelial activation important for development of numerous clinically different pathologic states. Major mechanisms of ED include: oxidative stress, endothelial NO synthase (eNOS) uncoupling, inflammatory pathways activation, differential expression of vascular endothelial growth factor, intracellular sorbitol accumulation. ED is a hallmark of impaired vascular integrity from which chronic complications of diabetes mellitus type 2 (DMT2) stem from. ED starts to appear in pre-diabetic state and worsens proportionally with duration of DMT2. Assessment of endothelial function in patients with risk of cardiovascular disease (CVD) development is of great importance because it enables us to detect vascular abnormalities and efficiency of therapeutic approach. Increasing quantities of therapeutic modalities have been shown to improve endothelial dysfunction, which has important implications for the treatment of patients at risk of developing chronic complications related to DMT2.