Ecotoxicology is the science of toxic substances in the environment and their impact on living organisms. Today we use many more chemicals in everyday life than we did 30-40 years ago. Our knowledge of the fate and effect of such chemicals in the environment has not yet followed the rate of chemical innovation in spite of our expanding knowledge of ecotoxicology. About 50,000 different chemicals are produced on an industrial scale, but we have only sufficient data to evaluate the environmental consequences of a few per cent of these. The need for ecotoxicological knowledge has never been more pronounced than it is today. Even more resources must be allocated in this field in the near future, if we are to be able to cope with the threat of more toxic chemical compounds in our environment.This book outlines the state of the art of modelling the fate and effects of toxic substances in the environment. Modelling in ecotoxicology differs from modelling in other fields by the great lack of data. The quality of the models is very dependent on the parameters used, and as we do not have a wide knowledge of parameters in ecotoxicological processes, good parameter estimation methods are crucial for ecotoxicolocal models. A comprehensive review of available parameter estimation methods is therefore included in this volume. Model examples and case studies have also been included to illustrate the difficulties and short comings in practical modelling.