Nitrogen is an important nutrient for the survival of all living organisms, as its chemical binding properties allow to create structures from which proteins are built. In nature, nitrogen is found in many forms and its transformation from one form to another is described by the nitrogen cycle. Ammonium is a form of nitrogen that has no direct effects on human health, but its presence in water supply systems may determine bacterial growth in the distribution networks and ultimately can generate degradation of the water quality. Its concentrations in wastewater and water intended for human consumption can be reduced by the biological process of nitrification. The purpose of this paper is to review the equations and the kinetics of the biological nitrification process.