During the last 25 years there has been a tremendous increase in observed data and knowledge about factors controlling trace gas fluxes from grasslands and agricultural systems. The observed data has shown that most of the annual N2O fluxes from grasslands and agricultural systems come from nitrification. Denitrification can make substantial contributions to annual N2O fluxes during the freeze-thaw time periods in the spring when the soil is frozen and snowmelt is occurring. Tillage of the soil as a result of agricultural practices can greatly reduce CH4 oxidation rates with a long lag time that can last as long as 50 years and soil water and temperature and soil texture have a substantial impact on controlling CH4 soil oxidation rates. The factors controlling NOx fluxes from the soil have been identified and include soil water content and temperature, soil texture and time since the last rainfall event. Computer models for predicting trace gas fluxes (N2O, CH4 and NOx) have been developed during this time period and have been used to predict the impact of management practices on trace gas fluxes and for estimating the nation's fluxes of trace gas fluxes. Arvin Mosier has been one of the leading scientists in collecting trace gas fluxes from agricultural and grassland systems during this time period and has worked extensively with computer modelers in developing state of the art trace gas flux models. His observed trace gas flux data sets have been used to test or formulate all of the existing trace gas flux models and are critical for our understanding about the factors that control trace gas fluxes in grassland and agricultural systems.