Acid deposition is considered to be one of the greatest threats to the environment in industrialised countries. Recognising this, governments have instituted programmes to reduce emissions of SO2, and NOx, the major sources of acid rain. But is this enough? Will ecosystems damaged by acid rain recover? And if they do, how quickly? What ecosystem processes determine the rate of recovery? Do acidification models give accurate predictions? Are any other actions necessary to promote recovery? To answer such questions without waiting for the results of current policies, scientists in Sweden and...