In the 1980s a continuous deterioration of crown condition of several forest tree species ‒ especially silver fir ‒ was observed. As air pollution ‒ mainly sulphur dioxide ‒ was taken as the main causal agent, an international initiative – ICP Forests – was launched in 1985 under the umbrella of the UNECE Convention on Long-range Transboundary Air Pollution (CLRTAP) in order to observe and evaluate forest tree crown condition. The early crown condition assessments, later called the Level I monitoring, were soon supplemented with data on the soil solid phase and foliar element content. A more intensive (Level II) monitoring was established in 1995 with the aim to gain a more thorough understanding of cause-effect relationships in forests. The Level II monitoring includes ‒ partly in high temporal resolution ‒ the collection of data on deposition, soil solution, weather condition, air quality, ground vegetation, tree increment, phenology, litterfall, and visible ozone injuries. The Programme Co-ordinating Centre (PCC) of ICP Forests is responsible for the programme administration and organization, and the data management. The PCC also edits the extensive ICP Forests Manual and publishes scientific papers and brochures for policy consultancy and the general public.