Ultraviolet degradation of polypropylene is reviewed with particular emphasis on the physical and engineering aspects. The depth profile of degradation is an important controlling factor and measurements are presented to show that it is changed markedly when a stabilizer is added. Laboratory exposure using UV radiation with a spectral distribution similar to that in solar radiation has been shown to produce similar changes in fracture mechanism as outdoor exposure in a hot climate. The ways in which molecular degradation leads to the deterioration of engineering properties are discussed with particular attention to the fracture mechanism, diffusion of oxygen, ultraviolet intensity and the ultraviolet transmittivity of polypropylene, the influence of stress, and the physical changes that follow from the chemical degradation, such as secondary crystallization.