Three numerical models of watershed acidification, including the MAGIC II, ETD, and ILWAS models, are reviewed, and a comparative study is made of the specific process formulations that are incorporated in the models to represent hydrological, geochemical, and biogeochemical processes which affect the responses of watersheds to acidic deposition. The models differ notably in the level of detail with which they represent various processes and in the degree of aggregation used to assimilate important watershed-specific parameters relating to hydrologic-flow routing and soil and solution chemistry. The models have been used to simulate the short-term dynamics of water quality at a number of different watersheds currently receiving acidic deposition. The predictive reliabilities of these models still need to be tested against observed data from watersheds that have been monitored for long periods.