Poly (vinyl chloride) (PVC) holds a major position in the consumption of thermoplastics. In the United Kingdom, from humble beginnings, 500 tons in 1941, production has increased rapidly through the last ten years to the 200,000 ton mark in 1966 and now accounts for 30 per cent of the total thermoplastics. This dramatic increase is a consequence of the polymer's outstanding properties and its diversity of application at low cost; it is now used in the form of pastes, latexes, solutions, plasticized and unplasticized films, moldings, and extrusions. Originally the advantage of PVC was its ease of processing with existing techniques and equipment developed mainly in the rubber industry, but as the application of PVC products expanded, so the processing conditions became more demanding and the stability of PVC at elevated temperatures became a problem. In recent years the potential of the rapidly expanding market for rigid unplasticized PVC products in the building industry has been moderated by the adverse weathering characteristics of the polymer and a solution of this problem is of immediate concern to the industry. When subjected to forms of energy, such as heat, light, and ionizing radiation, PVC liberates hydrogen chloride, with accompanying discoloration and a general deterioration of mechanical and electrical properties. Over the years several classes of materials, e.g., metal soaps, organo-tin compounds, and epoxides, have been successfully used to combat degradation and open the way to new applications of PVC products, but such stabilizers have been evolved empirically and, in order to make further improvements as regards efficiency, cost, and toxicity, basic knowledge of the mechanism of stabilization is desirable. At present it appears that an understanding of the stabilization mechanism requires detailed information concerning the mechanism of degradation. Thus a first logical step towards improving the stabilization of PVC is to determine the fundamental mechanism of degradation. It is possible, and indeed probable, that thermal degradation and atmospheric aging share the same mechanism, the difference being the form in which the stimulating energy is supplied. Photochemical reactions involving polymeric materials present a number of experimental difficulties for exhaustive quantitative study. In PVC degradation, most studies of a fundamental nature have been directed towards degradation at processing temperatures, which may seem paradoxical to PVC processors who claim to encounter no difficulties associated with stability during processing but do show concern over weathering and aging of their products. It is hoped that, once a mechanism has been evolved for thermal degradation, a mechanism for photochemical decomposition will follow by analogy. Several reviews of degradation and stabilization of PVC have appeared in the last few years but, in such an expanding and controversial field, reviews rapidly become out of date and a further review is therefore attempted here. Our purpose is to provide a critical assessment of work published within the last fifteen years and it is hoped that this will aid, in some small way, progress towards a universally acceptable mechanism of degradation.