The use of freshwater nematodes in the field of ecotoxicology and applied ecology is reviewed. After a general introduction to quality assessment of freshwater habitats, we try to elucidate the suitability of nematodes in this field of science and application. With examples of studies using nematodes as test organisms for single-species tests, as well as studies investigating nematode communities in unpolluted and polluted habitats, we highlight different fields of application and various methods for assessing the quality of ecosystems by focusing on studies performed in freshwater environments. © CAB International 2006.