This book deals in a concise format with 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 dynamic behaviour of reactors. In addition it includes an introduction into parameter identification, error analysis, error propagation, process control, time series analysis, stochastic modelling and probabilistic design. Written as a textbook it primarily supports graduate and doctoral students in environmental engineering but may serve as a resource for academics and practitioners alike.