While a large body of information is available on the environmental effects of parent chemicals, we know much less about the effects of transformation products. However, transformation products may be more toxic, more persistent and more mobile than their parent compound. An understanding of the ecotoxicity of transformation products is therefore essential if we are to accurately assess the environmental risks of synthetic chemicals. This chapter therefore uses data on pesticides and their transformation products to explore the relationships between parent and transformation product ecotoxicity to aquatic and terrestrial organisms and describes the potential reasons why a transformation product may be more toxic than its parent compound. As it is not feasible to experimentally assess the ecotoxicity of each and every transformation product, this chapter also describes and evaluates the use of expert systems, read-across methods and quantitative structure activity relationships for estimating transformation product ecotoxicity based on chemical structure. Finally, experimental and predicted ecotoxicity data are used alongside monitoring data for parent pesticides and their transformation products to illustrate how the risks of parent and transformation product mixtures can be assessed.