During the first half of the twentieth century, air pollution had a predominantly local character in Europe. But since the late 1960s, Europe has been aware of the threat to the environment from the effects of pollutants from large emission sources transported over long distances. At the end of the 1970s, the air pollution problem (mainly acid rain) was recognized as one of the most severe threats to the environment in Europe. As an effect of the alarming reports on the acidification problem the Convention on Long-Range Transboundary Air Pollution was adopted in 1979 within the framework of the United Nations Economic Commission for Europe (ECE). A first result of the implementation of this convention was an improvement of the Co-operative Programme for the Monitoring and Evaluation of the LongRange Transmission of Air Pollutants in Europe (EMEP). A second breakthrough for the implementation of the Convention was the Protocol on the Reduction of Sulphur Emissions or Their Trans boundary Fluxes by at least 30 per cent adopted in Helsinki in July 1985. A third important step in implementing the convention was a protocol conceiving control of emissions of nitrogen oxides, which was signed in Sofia in November 1988. The protocol, signed by 25 countries in Europe and North America, calls for a stabilization of the emissions of nitrogen oxides. In Sofia, 12 countries signed a declaration on a 30 per cent reduction of nitrogen oxide emissions. Thus the abatement of air pollution effects has a high ranking on the political agenda in many European countries.