The responses of forests to pollutants depend on the sensitivity of individual trees and species to the pollutants, and on complex interactions between trees, the environment and other organisms. Impaired growth in one individual or species may represent an opportunity for increased growth for other trees or species. An ecosystem perspective is necessary for a full assessment of pollutant impacts, but the response of forest to air pollutants remains poorly understood. In the chapter, the authors discuss important aspects of forests at the level of stands and ecosystems, as a foundation for the regional case studies presented in later chapters. Many key aspects of the effects of pollution at the stand and ecosystem level remain unexplored across the West; the chapter develops a framework for understanding pollution impacts, summarizes what is currently known, and provides some speculation where information is not available.