The importance of environmental photolysis of pesticides and other xenobiotics has been realized in the last decade and methods for assessing these processes are continually being improved. The general goal has been to develop quantitative laboratory procedures that can be used to estimate photolysis under a variety of environmental situations. To do this requires consideration of several factors involving both the chemical of interest and the environmental matrix. These include the absorption spectrum and quantum yield of the compound in each environmental matrix, the solar intensity at each wavelength, sunlight screening factors, and the contribution of indirect photochemical reactions to the dissipation of the xenobiotic. This paper reviews reported studies of photolysis of pesticides in air, on soils, and in water. Methods for assessing both the direct and indirect photolysis of pollutants in aqueous systems have received the most scientific attention. Studies under laboratory light systems and outdoor experiments are reviewed.