The increase of worldwide population and the need to control pests are some of the factors that have led to the application of agrochemicals on agricultural areas to protect and increase crop production. Nevertheless, these substances are of environmental concern since they can reach water reservoirs and act on non-target organisms. Therefore, different aquatic species have been tested to evaluate their sensitivity to different toxicants, including pesticides, so as to elucidate the secondary effects of these chemicals to estimate safe levels in aquatic media. " wide variety of toxicity tests can be found in literature to evaluate the toxicity of xenobiotics in the environment at organismal and sub-organismal levels under different regimes. This chapter focuses on those tests performed with some freshwater invertebrates cladocerans and rotifers to study the toxicity of four important classes of pesticides.