The ecotoxicological risk concept is considered to be the basis for setting environmental quality objectives for pesticides in groundwater. Ecotoxicological critical concentrations for groundwater can be derived through several exploratory approaches. These approaches will be subsequently compared (focusing mainly on the Netherlands but bringing the European Union standard for pesticides in groundwater into the relevant context ) with critically reviewed groundwater monitoring data. Furthermore, aldicarb, atrazine and MITC results of geographic modelling are ecotoxicologically evaluated. The study focuses mainly on the Netherlands but the European context is not forgotten. Many reported groundwater monitoring data are strongly correlated. Therefore the assessment of the extent of groundwater contamination should be based on the number of localities where pesticides have been detected. Shallow Dutch groundwater monitoring data reveal that aldicarb, 1,3-dichloropropene, 1,2-dichloropropane, dinoseb, dinoterb, ethoprophos and MITC are ecotoxicologically the most hazardous pesticides. Of the pesticides found in monitoring programs critical levels are lower than 0.1 4g/l for aldicarb and aldicarb sulfoxide, dinoseb, dinoterb, ethoprophos, heptachlor, MITC, parathion ethyl and pirimicarb. If ecotoxicological risk levels for groundwater are desired they should be derived from standard aquatic toxicity data, and the ecological characteristics of the system should be taken into account in the extrapolation. It was not found meaningful to develop ecotoxicity tests based on specific groundwater organisms.