The main aim of this new book is to summarize the knowledge on the metabolic transformation of carnosine in excitable tissues of animals and human beings and to analyze the nature of its biological activity. At the beginning of monograph, the short history of the problem is stated. Distribution of carnosine in tissues, its appearance in ontogeny of vertebrates and correlation between carnosine content and functional activity of tissues are discussed. Chemical properties of carnosine and its natural derivatives and their ability to bind heavy metals and protons in water solution are documented. Special attention is paid to free radical quenching ability and to anti-glycating action. Biological activity of carnosine and carnosine containing compounds was tested using biological models of several levels of complexity, starting from individual enzymes and acellular mixtures and finishing to living cells and survival animals. Effects of carnosine on the whole animals under ischemic, hypoxic and other extreme conditions are described. In conclusion, the ability of carnosine to protect brain and muscular tissues from oxidative injury during exhausting exercise, extreme loading or neurodegenerative diseases is demonstrated. Based on these properties, carnosine is postulated to be a potent protector of human beings from oxidative stress.