Endothelium has many important functions including the control of blood-tissue permeability and vascular tonus, regulation of vascular sur- face properties for homeostasis and inflammation. Nitric oxide is the chief molecule in regulation of endothelial functions. Nitric oxide de- ficiency, which is also known as endothelial dysfunction, is the first step for the occurrence of many disease states in cardiovascular sys- tem including heart failure, hypertension, dyslipidemia, insulin resistance, diabetes mellitus, hyperhomocysteinemia and smoking. This re- view deals with the importance of nitric oxide for cardiovascular system. It also includes the latest improvements in the diagnosis and tre- atment of endothelial dysfunction.