This chapter discusses developments in the anammox process, since its discovery, including identification of responsible microbes and their physiology, potential process inhibitors, reactor types used, and application to treat different wastewater. Anammox bacteria are coccoid bacteria with an average diameter ranging between 800 and 1100 nm. Anammox bacteria are chemoautotrophic bacteria and use CO2 as the main inorganic carbon source to oxidize ammonium to nitrogen gas using nitrite as the electron acceptor. Ammonium and nitrite are the two substrates for anammox bacteria. The absence of nitrite in wastewater limits the direct application of the anammox process. Applications of anammox or its related processes, such as SHARON-Anammox, CANON, or SNAD have been tested to treat real world wastewaters by many researchers in lab scale reactors. Anammox combined with partial nitrification has been identified as the most efficient, economical, and environmentally friendly nitrogen removal method in recent years.