Wastewater treatment is an increasingly costly business, so project teams should use the most proven, advanced tools. This includes the modeling tools used at the beginning of a project. For years, engineers used simple, steady-state models when designing wastewater treatment plants. These tools, which included “safety factors” to account for unknowns, were developed when construction costs were lower and federal grants often subsidized local utility costs. The safety factors, which often are justified as “engineering judgment,” simply incorporate historical presumptions about unknowns and design criteria. Some of the unknowns are created by the design tools themselves. They could be removed by using more advanced tools, such as dynamic models. While both steady-state and dynamic models are based on the fundamental principles of physics, chemistry, and biochemistry, dynamic models account for variations over time. This ability can significantly affect treatment plant designs, capacity ratings, and system optimizations. Dynamic modeling could cut costs and improve performance.