Steady-state models are often used for wastewater treatment designs, but have severe limitations. The use of safety factors does not fully account for the unknowns and lack of information that result from the limitations of the steady-state modeling approach. Dynamic modeling provides considerably more information than is possible with a steady-state model. This is important for optimizing secondary systems, particularly to meet stringent effluent limits and to evaluate secondary clarifier performance under peak wet weather flows. The validity of dynamic models depends largely on accurate calibrations conducted by experienced process engineers. Dynamic modeling represents the highest level of consultant service for municipal clients because it allows for the best predictions of system performance and allows the best value for capital and operating expenditures. This paper presents several examples where dynamic modeling has provided valuable information that could not have been obtained if steady-state modeling had been used.