Endothelial dysfunction (ED) is a pathological condition that is impairment in the synthesis of endothelial factors and is a major component in the pathogenesis of many diseases, including those of a therapeutic profile. Many specialists now consider ED as a universal component in the development of cardiovascular and respiratory diseases. The paper contains information about the main functions of the endothelium and their impact on the organs and systems of the body, the state of microhemocirculation, and the entire internal environment. It also considers ED that affects vasomotor, hemostatic, adhesive, and angiogenic functions and the main markers of endothelial damage. The possibilities of preventive measures and pharmacological correction of ED are discussed.