Technical Abstract: Greenhouse gas (GHG) emissions and their potential impact on the environment have become important national and international concerns. Dairy production, along with all other animal agriculture, is a recognized source of GHG emissions, but little information exists on the net emissions from our farms. Component models for predicting all important sources of methane, nitrous oxide, and carbon dioxide emissions from primary and secondary sources in dairy production were integrated in a software tool called the Dairy Greenhouse Gas Model or DairyGHG. This tool calculates the carbon footprint of a production system as the net exchange of all GHGs in carbon dioxide equivalent (CO2e) units per unit of milk produced. Primary emission sources include enteric fermentation, manure handling facilities, cropland used in feed production, and the combustion of fuel in the machinery used to produce feed and handle manure. Secondary emissions are those occurring during the production of resources used on the farm, which can include fuel, electricity, machinery, fertilizer, pesticides, plastic, and purchased replacement animals. A long-term C balance is assumed, which does not account for potential depletion or sequestration of soil carbon. Depending upon farm size, milk production level, and the feeding and manure handling strategies used, the carbon footprint of production systems was found to range from 0.4 to 0.8 kg CO2e per kg of milk produced. This footprint was most sensitive to the amount of methane produced through enteric fermentation, moderately sensitive to the GHG emissions during long-term manure storage, and mildly sensitive to the amount of fuel, electricity and inorganic fertilizer used on the farm. DairyGHG provides a relatively simple tool for evaluating management effects on net GHG emissions and the overall carbon footprint of dairy production systems. This tool is available at http://www.ars.usda.gov/Main/docs.htm?docid=17355 for download and installation on computers using Windows operating systems.