This chapter gives an overview of tertiary treatment processes, which are advanced methods used after primary and secondary treatment to further improve wastewater quality. The objectives of tertiary treatment, including nutrient removal, pathogen reduction, and removal of residual suspended solids, trace organics, and emerging contaminants are outlined in this chapter. Key technologies discussed include filtration (sand, dual-media, membrane systems), disinfection (chlorination, ozonation, ultraviolet), nutrient removal methods (biological and chemical), and advanced oxidation processes. The chapter also addresses process selection criteria, operational considerations, and performance efficiencies. Emphasis is placed on the role of tertiary treatment in meeting stringent regulatory standards, protecting sensitive receiving waters, and enabling water reuse. Challenges such as cost, energy use, and sludge management are also highlighted, alongside trends toward integrated, sustainable treatment solutions.