This paper deals with freshwater model ecosystems as tools for assessing the potential hazards of pesticides in aquatic ecosystems. Examples are given of the types of information that can be obtained with these test systems. The advantages and drawbacks of model ecosystems are discussed. It is concluded that model ecosystems are capable of providing valuable data for hazard assessment of pesticides, particularly to assess factors that determine the fate of pollutants, to validate the significance of single-species toxicity tests, to gain insight into secondary (indirect) effects, and to assess the (potential) recovery of populations of species affected by pesticide contamination