The arbuscular mycorrhiza (AM) is characterized by specific morphological structures of the fungus and the plant and by physiological adaptations which are mostly beneficial for both partners of the symbiosis. This review describes approaches to study the molecular basis of the interaction. RNA accumulation patterns have been monitored in Pisum sativum to analyse the plant response to arbuscule development. In a direct approach, the Mtha1 gene from Medicago truncatula was cloned which is expressed in arbusculated cells of M. truncatula. The gene putatively encodes an H(+)-ATPase involved in the improved plant nutrition during mycorrhization. Finally, a tripartite system between M. truncatula, Glomus mosseae and Aphanomyces euteiches was established, in order to study bioprotection. Analysis of the transcriptome has been started to analyse the interaction between the plant, the pathogen and the AM fungus.