Air quality and ecosystem monitoring and research indicate that anthropogenic nitrogen pollution is beginning to alter sensitive ecosystems in the Greater Yellowstone Area (GYA). Some GYA lakes may be at the early stages of eutrophication (nitrate concentrations are at levels where algal species may increase), and some lakes are beginning to acidify (lose acid neutralizing capacity).  Lake sediment cores show increasing influences of anthropogenic nitrogen and degraded lichen communities are present in areas of higher nitrogen deposition. Although the ecosystems changes are subtle, an increasing weight of evidence points to declining health in aquatic and terrestrial ecosystems in the GYA. Deposition and ambient air monitors also indicate that nitrogen compounds in air, rain, and snow are increasing in several areas of the GYA. Critical loads indicating thresholds of change for chemical and biological endpoints have been developed to show the levels of nitrogen specifically impacting different ecosystem components in the GYA. Critical loads in the GYA can: (1) Help National Park and National Forest land managers set goals to protect and improve resource conditions and (2) Serve as benchmarks identifying areas and pollutants for which State and Regional Plans to improve air quality would be most effective.