COST (Cooperation Scientifique et Technique) Networks or Actions were initiated in 1971 by the European Union. The aim of COST Actions is to coordinate on a volontary base, precompetitive scientific and technical research financially supported by each individual COST country. Today there are 25 countries that have jointed COST. The first COST Action on « Arbuscular mycorrhizas » started in 1989 followed in 1994 by a second one entitled « Arbuscular mycorrhizas in sustainable soil/plant systems». The latter is structured in five working groups (1- Mycorrhizal fungi: biodiversity and population dynamics; 2- Genetics and physiology of mycorrhizal systems; 3-Mycorrhiza in abiotic and biotic stress alleviation and plant protection; 4- Mycorrhiza in sustainable systems in relation to biogeochemical cycling; 5- La Banque Européenne des Glomales) and laboratories of 22 countries participate to their activities. COST provides therefore an official and very efficient forum to present and discuss european advances in mycorrhizal research.