The amount of European level organized interest groups has dramatically increased since the Single European Act came into force in 1987. In 1995 approximately 3000 lobby-groups, employing 10.000 lobbyists, are based in Brussels. This has resulted in a complex arrangement of European interest intermediation and caused important constraints on decision making procedures and institutional actors. The first part of this article offers a discription and evaluation of this proliferation and diversification of European interest groups. It alsosuggests a classification of all European interest groups including professional lobbyists and actors that defend their interests by themselves. The second part gives a discription of the way in which both European Parliament and Commission have dealt with this proliferation and assesses the differences in approachbetween both institutions.