The main purpose of secondary treatment (sometimes referred to as biological treatment) is to provide biochemical oxygen demand (BOD) removal beyond what is achievable by primary treatment. Three commonly used approaches all take advantage of the ability of microorganisms to convert organic wastes (via biological treatment) into stabilized, low-energy compounds. Two of these approaches, the trickling filter or its variation, the rotating biological contactor (RBC), and the activated sludge process, sequentially follow normal primary treatment. The third approach, ponds (oxidation ponds or lagoons), however, can provide equivalent results without preliminary treatment. In this chapter, we present a brief overview of the secondary treatment process followed by a detailed discussion of wastewater treatment ponds (used primarily in smaller treatment plants), trickling filters, and RBCs. In the next chapter, we shift focus to the activated sludge process, the secondary treatment process that is used primarily in large installations and is the main focus of this handbook.