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Acidification of lakes and streams from long-range transboundary air pollution is one of the most severe and spatially extensive environmental problems in northern Europe and North America. The Nordic countries, with acid sensitive soils and located downwind of the industrial areas of western and central Europe were particularly affected, with local extinctions of fish populations and other harmful effects on the aquatic ecosystems. Although the deposition of acidic pollutants today is tenfold lower than during peak years in the 1980s, acidification is still a major problem due to legacy acidification of the soils in the catchments of lakes and streams. The Nordic countries have developed different criteria to classify acidification from chemical parameters and to distinguish anthropogenically acidified waters from naturally acidic waters. In brief, the different systems reflect dissimilarities in geology and climate and different forms of management. This has resulted in acidification assessments that are not directly comparable. In international reporting, for example to the UN-ECE Air convention and the EU Water Framework Directive, discrepancies among the Nordic countries reflect more the different classification systems used rather than environmental conditions. To address this issue, the Swedish Agency of Marine and Water Management and the Norwegian Environment Agency initiated a project to assess the possibility of harmonising classifications of acidification across the Nordic countries, as well as to lay a foundation for improved and harmonised systems and reporting. The project focused on analyses of a joint database, comprising data on water …