The purpose of the present series of publications is two-fold. In the first place it is intended to review progress in the development of practical stabilising systems for a wide range of polymers and applications. A complementary and ultimately more important objec tive is to accommodate these practical developments within the framework of antioxidant theory, since there can be little question that further major advances in the practice of stabilisation technology will only be possible on the basis of a firm mechanistic foundation. Research into the role of ‘stable’ free radicals as antioxidants and stabilisers for polymers has intensified in recent years. Nitroxyl radicals (nitroxides) were the earliest long-lived radicals to be investi gated in detail and Maslov and Zaikov review the developments that have taken place in understanding their reaction mechanisms from the time when they were first investigated in liquid hydrocarbon systems to the present day when their outstanding performance as light stabilisers has been the object of much scientific research. Although some features of their reactivity remain obscure, the authors approach the problem kinetically and indicate the factors limiting their effectiveness.