AbstractThis chapter provides an overview of the use of nematodes as test organisms for ecotoxicity testing. Exemplary studies that investigated the toxicity of single substances or environmental samples on nematodes, regarding different nematode species, toxicity endpoints and test substrates are reviewed. The main focus is on the most frequently used species, Caenorhabditis elegans, by presenting examples and case studies, where this test organism has been used in ecotoxicological assessments.