Four soil nitrogen dynamics models are reviewed and compared. These models are SOILN from Sweden, ANIMO from The Netherlands, DAISY from Denmark and SUNDIAL from the UK. The constituent processes are analysed with particular reference to the equations used in each of the models. Processes considered are surface application (as fertilizer, manure or slurry, atmospheric deposition and deposition or incorporation of dead plant material), mineralization/immobilization (between organic and inorganic forms), nitrification (from ammonium to nitrate), nitrate leaching, denitrification (to N2O and N2) and uptake by plants. Sources of information for model parameters are also reviewed and compared with values assumed in the models. Rate coefficients are comparable for most transformation processes, except denitrification where a further more detailed study is required. Assumptions about the effects of temperature and soil water content on transformation rates are compared, and shown to be broadly similar in all models. Previous applications of the models have been mainly for arable land receiving mineral fertilizer; an assessment of their applicability to grassland receiving animal manure and slurry suggests that they are suitable for this application with only minor modifications.