During the last 10 years, there has been a `revolution' in ecosystem modelling. The generality and predictive power of our models have increased in a way that was inconceivable 10 years ago. This book describes a new generation of practically useful models that predict as well as one can measure - if one measures well. And yet, they are driven by readily available driving variables and have a general structure that applies to most types of pollutants in aquatic systems. The major reason for this development is, in fact, the Chernobyl accident. Large quantities of radiocesium were released in April/May 1986 as a pulse. To follow the pulse of radiocesium through ecosystem pathways has meant that important fluxes and mechanisms, i.e., ecosystem structures, have been revealed. It is important to stress that many of these new structures and equations are valid not just for radiocesium, but for most types of contaminants, e.g. for metals, nutrients and organics. This means that the models, methods (of building and testing models) and equations described in this book for lakes and coastal areas should be of great interest also to other ecosystem modellers. This book will be of considerable interest to: students in radioecology, geosciences and biology; environmental engineers; consultants; administrators and scientists interested in the spread, biouptake and ecosystem effects of chemical pollutants in aquatic ecosystems.