Twenty years after acid rain captured headlines and forced governments to act, acid deposition has now declined by ∼60% in Europe. Some lakes and streams have even begun to recover. Richard F. Wright and 13 colleagues from research institutes in Europe and the United States use models of 12 acid-sensitive regions to predict the continuing effect of declining acid deposition. Will all waters recover, or do acid-rain-causing emissions need to be reduced even more? And how long will the recovery take?