This work contains 26 contributions dealing with the biology of aquatic oligochaetes and covers a wide range of topics including taxonomy, morphology, ultrastructure, embryology, reproduction, feeding biology, ecotoxicity, community studies, and species distribution. Descriptions of new taxa in tropical areas, including Amazonian forest soils, as well as overviews on the biodiversity of aquatic oligochaetes in Australia and European groundwaters, are presented. New morphological characteristics in both marine and freshwater species are described and interpreted. Laboratory studies contribute to the knowledge of oligochaete feeding biology and reproduction. The use of aquatic oligochaetes in ecological risk assessment is analysed in detail, and standardised experimental designs for studies on bioaccumulation and pollutant transfer by food are included. Finally, a number of papers present the effects of oliogochaetes on the performance of an activated sludge plant, and multivariate approaches to the spatial and/or temporal distribution and composition of oligochaete communities in many different areas of the world, from the scale of a river to the scale of the microhabitat. The broad scope of this volume is a reflection of recent rends, not only in oligochaete research, but also in general applied biological studies.