We provide an assessment of the global sources of NH3 in the atmosphere, which indicates an annual flux of about 75 Tg of N as NH3. The emissions from land are dominated by the release of NH3 during the hydrolysis of urea from the urine of domestic animals (32 TgN/yr) and by emanations from soils in unmanaged ecosystems (10 TgN/yr) and from fertilized agricultural soils (9 TgN/yr). Emissions from the sea surface may approach 13 TgN/yr. The total annual source of NH3 is in reasonable agreement with estimates of global NH4+ deposition from the atmosphere, the major fate of atmospheric NH3. As an alkaline atmospheric species, NH3 emitted to the atmosphere each year can neutralize only about 32% of the annual production of H+ in the atmosphere from natural and anthropogenic sources.