The acidification of lakes and streams due to acid precipitation has been documented in southern Sweden and Norway, the northeastern United States and southern Ontario. Geochemistry and regional lithology are recognized to be important factors in the susceptibility of lake ecosystems to acid precipitation. Local soils, glacial deposits and bedrock undergo biological and chemical weathering reactions to consume protons and release base cations - the greater is the total alkalinity production in the watershed, the greater is the capacity to neutralize acid loadings. Acid loadings are composed of wet and dry deposition to a watershed, and these are also important factors in the degree of acidification of lakes. A schematic of the trickle-down model, which is used in acid precipitation assessments, is shown. The hydrology and geochemistry of the watershed determine the chemical weathering rate and thus are key factors in the susceptibility of lakes to acidification. In this chapter, lakes in northeastern Minnesota serve as case studies, where igneous bedrock and a lack of calcareous overburden are sufficient to classify the region as sensitive to acid rain. The volume-weighted acidity of precipitation pH ranges 4.6-4.85. These are threshold cases where it is not certain whether present acid loadings are acidifying lakes.