Nitrification and denitrification are essential processes for the aquatic ecological system and vital for human health. While ammonia is applied for disinfection together with chlorine to produce chloramine, excessive ammonia may cause nitrification and bacteria growth in the water transmission pipeline. Since excessive discharge may cause eutrophication and deterioration of the aquatic system, nitrate is regulated for wastewater discharge in sensitive areas. Further, nitrate needs to be monitored and controlled in drinking water treatment to protect against methemoglobinemia in bottle-fed infants. Various conventional technologies exist, such as adsorption, ion exchange, photocatalytic oxidation, air stripping, biological nitrification and denitrification, and so on, to remove nitrogenous compounds from water. Since ammonia and nitrate are important constituents in fertilizers besides phosphorus (P) and potassium (K), nutrient recovery is drawing attention to maintaining the supply of reliable and sustainable fertilizers. This book provides a comprehensive overview of nitrification and denitrification.