The use of models to assess water quality is becoming increasingly important worldwide. In order to be able to develop a good model, it is necessary to have a good quantitative and ecological description of physical, chemical and biological processes in ecosystems. Such descriptions may be called ``submodels''''. This book presents the most important, but not all, submodels applied in water quality modelling. Each chapter deals with a specific physical process and covers its importance, the most applicable submodels (and how to select one), parameter values and their determination, and future research needs.The book will be an excellent reference source for environmental engineers, ecological modellers and all those interested in the modelling of water quality systems.