Most anthropogenic ammonia in the atmosphere is emitted by agriculture. The principle sources are animal manures and ammonia based fertilizers. A detailed inventory was developed in Canada to quantify on-farm sources in each of the main livestock and crop sectors. The inventory was based on published emission factors, census numbers for livestock and specially designed farm surveys for quantifying ammonia-relevant practices and facilities across Canada. Total annual Canadian emission of ammonia is bout 400 tonnes. Ammonia is emitted from barns and open housing, pastures, manure storages, and land application of manure and fertilizers. This inventory has identified key on-farm sources for each sector and suggests effective mitigation measures to reduce emissions. The inventory also desegregated emissions by time and space allowing for more accurate input into atmospheric models. These models help to predict the atmospheric transformations and long range transport of ammonium, and patterns of deposition that can affect air quality, natural vegetation and species diversity. The emission inventory will also be used for calculating agricultural N budgets in 3000 agricultural polygons across Canada and for a national budget of reactive N species from all sources.