Once applied in the environment, an agrochemical compound is exposed to direct or diffused sunlight, as a result of which it undergoes photolysis rapidly or slowly, and often produces multiple degradation products. To understand the fate of the agrochemical in the environment and to resolve the risk associated with its use, regulatory agencies require that photolysis studies be submitted as a condition of registration. Photolysis studies are carried out using a tiered approach by exposing the radiolabeled agrochemical to actual or simulated sunlight under abiotic conditions. Direct photolysis tests are carried out in irradiated sterile buffered aqueous solutions at 25 °C with non-irradiated controls for a period of 30 days, or earlier when 50% photolysis is achieved. The photolytic rate constant and half-life of the test compound and the identity of its significant degradation products are determined from these tests. The procedure to calculate the seasonal quantum yield (for summer and winter) and direct sunlight photolysis rate constant in water bodies in the environment using the molar UV/visible absorption coefficients and photolytic rate constants of a test agrochemical is discussed. The photolytic pathways of major groups of agrochemicals under various test conditions are also discussed in this chapter.