The eleven western states occupy about 40% of the land area of the conterminous United States. The West is characterized by extreme relief with massive north-south oriented mountain ranges separated by basins, valleys, and plateaus. This physiography is an important determinant of the exposure and response of western forests to air pollution. Topographic modification of regional climate determines which areas can support forests. Patterns of relief influence the distribution of pollutant sources relative to forests and the transport of pollutants from source areas, so pollutant loading to western forests is also influenced by topographic patterns. The chapter presents a broad overview of western physiography and forest types as a framework for later chapters. Discussion of patterns within forest types focuses on topographic and moisture relationships.