This chapter presents the main photoinduced transformation processes involving pesticide molecules in sunlit surface waters, on soil, and in the atmosphere (taking into account both the gas and the condensed phases). Moreover, a recently developed model approach to predict pollutant phototransformation in surface waters is presented and described together with an example referred to the herbicide MCPA (4-chloro-2-methylphenoxyacetic acid) and to the production of the toxic transformation intermediate 4-chloro-2-methylphenol.