Biodegradation is a key phenomen among environmental processes. Low degradation rates lead to the persistence of chemicals in the environment and, as a consequence, to delayed or long-term effects, which may be even unknown by now. In this volume the editor has pulled together the newest results of research in biodegradation and persistence of potential environmentally harmful substances and the complex process involved. The main focus is on the microbial degradation, the evolution and predictability of the respective pathways and their impact on bioremediation. Additional chapters deal with sewage treatment plants, the impact of toxicants on impaired biodegradation, and with the need of a more realistic view on fate and behaviour of chemicals in the environment.