Recent intensive study of the causes of surface water acidification has led to numerous hypothesized controlling mechanisms. Among these are the salt-effect reduction of alkalinity, the base cation buffering and sulfate adsorption capacities of soils, availability of weatherable minerals, depth of till, macropore flow, and type of forest cover. Correlative and predictive models have been developed to show the relationships(if any) between the hypothesized controlling mechanisms and surface water acidity, and to suggest under what conditions additional surface waters might become acid. The US EPA is interested in surveying watershed characteristics to correlate with predictive model simulations in and effort to assess how many surface waters will become acid within certain timeframes. The document is a review of our current knowledge of factors and processes controlling soil and surface water acidification, as well as an assessment of the adequacy of that knowledge for making predictions of future acidification.