Chronic acidification of surface waters refers to loss of acid neutralizing capacity or reduction in pH on a chronic, or annual-average, basis. Chronic acidification is often evaluated by studying changes in surface water chemistry during periods when that chemistry is expected to be relatively stable. There has been much advancement in the scientific understanding of chronic surface water acidification since 1990. A major research effort was conducted in Europe regarding the dynamics of N-driven acidification and related processes in both terrestrial and aquatic ecosystems. The western US contains many of the surface waters most susceptible to potential acidification effects, but the levels of acidic deposition in the West are generally low and acidic surface waters are rare. The Shenandoah National Park—Fish in Sensitive Habitats Project documented adverse effects on fish populations and communities in chronically acidified streams of Shenandoah National Park.