Technical Abstract: The Dairy Gas Emissions Model (DairyGEM) is a software tool for estimating ammonia, hydrogen sulfide, and greenhouse gas (GHG) emissions of dairy production systems as influenced by climate and farm management. A production system is defined to include emissions during the production of all feeds whether produced on the given farm or elsewhere. DairyGEM uses process level simulation to predict ammonia and hydrogen sulfide emissions from manure in the housing facility, during long term storage, following field application, and during grazing. Hourly and daily emission rates from each phase are predicted as a function of temperature, manure pH, manure nitrogen contents, air velocity, and other factors. Process-based relationships and emission factors are used to predict the primary GHG emissions from the production system. Primary sources include the net emission of carbon dioxide plus all emissions of methane and nitrous oxide during feed and animal production processes. All emissions are predicted through daily simulations of feed use and manure handling. The net GHG emission is determined through a partial life cycle assessment of the production system including secondary sources that occur during the manufacture or production of resources used in the production system. These resources include machinery, fuel, electricity, fertilizer, pesticides, plastic, and any replacement animals not raised on the farm. By totaling the net of all annual emissions from both primary and secondary sources and dividing by the annual milk produced (corrected to 3.5% fat and 3.1% protein), a carbon footprint is determined in units of CO2 equivalent per unit of energy corrected milk. DairyGEM is available through Internet download at https://www.ars.usda.gov/Main/docs.htm?docid=21345. This Windows-based software tool is made available as an educational aid for assessing the impacts of strategies used to mitigate gaseous emissions from dairy farms.