Several different types of models which can be useful in describing the dynamic behavior of wastewater treatment plants are discussed. Included among these are (1) visual, (2) linguistic, (3) mental, (4) physical, (5) mathematical, and (6) fuzzy models. Some of the basic concepts of modeling with emphasis on the relationship of these to engineering research and practice are then presented, these being (1) differences in the use of, and expectations from, models by researchers and practitioners, (2) sources of information useful for model development, (3) the amount and type of testing needed for model validation, and (4) the required accuracy of models. Computer simulation is necessary for the solution of most dynamic models. Recent advances in both hardware and software for personal computers have resulted in inexpensive, user-friendly systems suitable for use in both large and small organizations as well as by individual engineers.