The book comprises four parts. The first part (Chapters 1-3) deals with background issues relating to nematodes themselves and to bioindicators in general. Part II (Chapters 4-8) describes practical aspects of using nematode assemblages as bioindicators in real world settings. Part III (Chapters 9-11) describes the potential of nematodes, particularly the model organisms Caenorhabditis elegans, to be used in laboratory-based toxicity assays. It is precisely the same features (short life span, ease of culture, transparent cuticle) that made C. elegans the first model animal that make it so suitable for laboratory-based bioassays. Furthermore, because of the vast amount of knowledge and technical know-how generated by the C. elegans community, data can be generated and interpreted from C. elegans assays better than from any other animal-based biosensor. The final part (Chapter 12) deals with commercial aspects of using nematodes as environmental bioindicators.