Nitrifier denitrification is the pathway of nitrification in which ammonia (NH3) is oxidized to nitrite (NO2−) followed by the reduction of NO2− to nitric oxide (NO), nitrous oxide (N2O) and molecular nitrogen (N2). The transformations are carried out by autotrophic nitrifiers. Thus, nitrifier denitrification differs from coupled nitrification–denitrification, where denitrifiers reduce NO2− or nitrate (NO3−) that was produced by nitrifiers. Nitrifier denitrification contributes to the development of the greenhouse gas N2O and also causes losses of fertilizer nitrogen in agricultural soils. In this review article, present knowledge about nitrifier denitrification is summarized in order to give an exact definition, to spread awareness of its pathway and controlling factors and to identify areas of research needed to improve global N2O budgets. Due to experimental difficulties and a lack of awareness of nitrifier denitrification, not much is known about this mechanism of N2O production. The few measurements carried out so far attribute up to 30␘f the total N2O production to nitrifier denitrification. Low oxygen conditions coupled with low organic carbon contents of soils favour this pathway as might low pH. As nitrifier denitrification can lead to substantial N2O emissions, there is a need to quantify this pathway in different soils under different conditions. New insights attained through quantification experiments should be used in the improvement of computer models to define sets of conditions that show where and when nitrifier denitrification is a significant source of N2O. This may subsequently render the development of guidelines for low-emission farming practices necessary.