This review deals with the microbiological and chemical phenomena involved in the denitrification and N2O emission processes in soils. The different methods of measurement, the major results of in situ measurements, and predictive models of denitrification and N2O emissions are discussed. Important areas of future research including methodological aspects, the need for knowledge concerning the spatial variability of high denitrification or N2O production activities, and the impact of agricultural practices on these processes are described.