Ammonia (NH3) is the dominant gaseous base in the atmosphere and the principal neutralizing agent for atmospheric acids, yet remains one of the least well characterized atmospheric trace compounds. In particular, the spatial and temporal distribution of the background concentrations in terrestrial ecosystems and the importance of natural emissions from undisturbed soils and vegetation is poorly understood. This situation persists because of experimental difficulties associated with ammonia measurements, the rapid gas-to-particle conversion of ammonia in the atmosphere, and the capacity of native soils and vegetation to act as both source and sink for atmospheric ammonia. In the present paper, we attempt to summarize the current understanding of the natural sources and sinks for gaseous NH3 and the importance of natural emissions relative to anthropogenic emissions in the United States. We briefly review the physical and chemical processes that transform NH3 in the atmosphere, the major anthropogenic and potential natural sources of atmospheric NH3, and the techniques used to measure low concentrations and fluxes of atmospheric NH3. The available background concentrations and flux measurements of ammonia in natural ecosystems are then described and used to infer upper limits for the emissions of NH3 into the atmosphere from these systems. While the magnitude of both anthropogenic and natural emissions of NH3 remain uncertain, it appears that unperturbed terrestrial ecosystems are generally more important as sinks rather than sources for atmospheric NH3. However, net emissions are likely from many eastern forests and other ecosystems exposed to large inputs of atmospheric sulfate.