Book excerpt: Nematodes are the most wide spread multicellular animals in nature and analysis of nematodes in terrestrial, freshwater and marine environments as well as their role and function in ecosystems, can be used for environmental monitoring. Compared to other organisms, they offer the greatest potential as bioindicators and can be used to study gene expression in relation to environmental challenges, to monitor changing impacts on the environment and in laboratory ecotoxicity tests. This volume addresses classical and molecular approaches to nematode community analysis, the contemporary field of nematodes as biosensors, as well as genomic aspects of nematode bioindicators. In addition, the case studies stress the importance of these bioindicators and demonstrate the commercial potential of these technologies.