This chapter describes the historical background of biological phosphorus removal, the fundamental biological mechanism responsible for biological phosphorus removal, the system designs, and critical process and design considerations that affect the performance of biological phosphorus removal systems. It suggests that J. L. Barnard proposed the use of a separate anaerobic basin ahead of the aerated activated sludge basin and termed the process the Phoredox process. The soluble biochemical oxygen demand concentration decreases in the anaerobic zone even though there is no aerobic or anoxic electron acceptor present. H. A. Nicholls and D. W. Osborn provided direction for further understanding and modification of the anaerobic-aerobic biological phosphorus removal system. They proposed a biochemical model involving carbon storage products, such as polyhydroxybutyrate, and polyphosphates to explain biological phosphorus removal. Microbiological literature indicates that a number of microorganisms are capable of storing excess amounts of phosphorus in their cells.