This book deals with mediation by nitric oxide (NO) of many biological functions. Nitric oxide, initially known as Endothelial Derived Relaxing Factor (EDRF, Furchgott, Zawadzki 1980), six years later was identified as nitric oxide. Nowadays, its significance reached clinical applications. Clinical significance of nitric oxide encompasses: a/ application of NO-donors such as nitroglycerin and other organic nitrates or sydnonimines in cardiovascular diseases b/ exploitation of stimulation of nitrergic nerves in bronchial asthma or treatment of male impotence c/ resuscitation of fading endothelial function in diabetes or atherosclerosis by using drugs which stimulate vascular endothelium d/ protection of vascular or cardiac transplants from rejection e/ regulation of renal function f/ management of septic shock patients, and many other applications. Basic research of NO is of utmost importance for its further practical applications. Among the authors of the book there are two Nobel Prize winners, Robert Furchgott and Murad Ferid, in physiology and medicine who received their prize in 1998 for the discovery of biological significance of nitric oxide. This work presents the latest data on: cellular signalling by nitric oxide, structure and activity of nitric oxide synthases, the mode of activation by nitric oxide of soluble guanylate cyclase, the role of formation of peroxynitrites, the mode of inhibition of nitric oxide synthase by drugs, interaction between nitric oxide and other endothelial mediators such as prostacyclin or plasminogen activator, specificity of physiology of isoenzymes of nitric oxide synthase in vascular wall, in brain and in immunological system. Practically, therecent achievements in nitric oxide basic research and therefrom deriving clinical applications - as seen from West and East Europe and the U.S.A. - will all be covered by the chapters of this book. In summary, this book will outline the hottest issues on biology, pharmacology, and biochemistry of nitric oxide, with a sound support from the latest clinical applications of the "NO principle".