This book presents in a concise format the methods used to develop mathematical models for water and wastewater treatment. It provides a systematic approach to mass balances, transport and transformation processes, kinetics, stoichiometry, reactor hydraulics, residence time distribution, heterogeneous systems, and the dynamic behaviour of reactors. In addition it includes an introduction to parameter identification, error analysis, error propagation, process control, time series analysis, stochastic modelling and probabilistic design. Written as a textbook it is primarily intended to support graduate and doctoral students in environmental engineering, but may also serve as a valuable resource for academics and practitioners alike.