This report is a technical description and a user-manual of the PERPEST model, able to Predicts the Ecological Risks of PESTicides in freshwater ecosystems. This system predicts the effects of a particular concentration of a pesticide on various (community) endpoints, based on empirical data extracted from the literature. The method that it uses solves new problems (e.g., what is the effect of pesticide A?) by using past experience (e.g., published microcosm experiments). The database containing the `past experience` has been constructed by performing a review of freshwater model ecosystem studies evaluating the effects of pesticides. The PERPEST model searches for situations in the database which resemble the question case, based on relevant (toxicity) characteristics of the compound.The model is described in the scientific paper written by Van den Brink et al. (2002) and available via the enclosed CD-ROM and the website www.perpest.alterra.nl