Ecological indicators are needed to improve understanding and monitoring of the effects of air pollutants on ecosystems and to scientifically assess the effectiveness of air pollution control strategies. Traditionally, research and monitoring of air quality have focused on human health impacts and have been concentrated in urbanized areas, however pollution impacts on ecosystems are an equally important measure of how well our emission control policies are working. Decision-makers need tools to help them understand whether and how their decisions are contributing to the achievement of air quality goals. Ecosystem monitoring is one such tool; chosen well, indicators of ecosystem change can help to inform policy development and implementation by documenting whether emission control policies and programs are working as intended and helping to determine if policy change is needed to achieve further ecosystem protection. With support from the U.S. Environmental Protection Agency (EPA) Clean Air Markets Division, the Heinz Center has developed a suite of indicators to inform environmental data collection and integrated assessment of ecosystem response to changes in air quality. This report focuses on four major ecological effects of air pollution: acidification by nitrogen and sulfur, nitrogen enrichment, ozone damage to plants, and mercury bioaccumulation.