The optimization and control of wastewater treatment plants (WWTP) is very challenging due to the microbiological nature of the processes involved. Since many years, researchers have developed many methodologies for giving a clean effluent, but nowadays, the importance of environmental aspects has increased, leading to the necessity of considering eco-efficiency and sustainability of the overall process operation. The Benchmark Simulation models No. 1 and No. 2 (BSM1 and BSM2) presented in this chapter are benchmark platforms that represent a standard activated sludge WWTP, including classical controllers for dissolved oxygen and nitrogen. In the last decades, they have been used extensively for academic research since they provide a comprehensive framework to test new optimization and control strategies. Currently they continue their popularity for the development of ground-breaking methodologies. The plant-wide layout is described, including an overview of the mathematical models involved, the simulation procedure, some evaluation criteria, and influent disturbances. BSM1 comprises the water line, with five biological reactors and the secondary settler, and BSM2 is a plant-wide platform that enlarges BSM1 with sludge treatment units and a primary clarifier. The chapter ends with some successful examples of application and current uses, introducing researchers into the field.