Chemiluminescence is the light emission accompanying some chemical reactions. Chemiluminescence accompanies energetic redox reactions, e. g. interaction of two radicals or reactions where peroxides are involved. Intrinsic ("ultra weak") chemiluminescence of human and animal cells and tissues is a result of reactions of free radicals, including nitric oxide and radicals of oxygen and lipids; all these substrates are vitally important for the organism life and, under certain conditions, responsible for development of a variety of pathological states.