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Over the last two decades strong evidence has emerged that. interactions between .soil microbes, plants and higher trophic levels can translate into functional changes which affect ecosystem functioning and productivity. One of the most intensively studied soil microbial groups are the arbuscular mycorrhizal fungi that have been demonstrated not only to affect the performance of plants, but also to interact with insect herbivores via the common host. Howeyer, the ecological significance of such interactions on above- and belowground processes often remain~ obscure, since most previous studies were conducted under the . exclusion ofnon-mycorrhizal soil organisms. In order to study mycorrhiza-insect herbivore interactions under ecologically more realistic conditions, the large majority of experiments presented in this thesis were carried out with naturally co-occurring soil microbial communities. Chapter three of this thesis describes experiments in which I examined the effects of insect shoot herbivory on mycorrhizal colonisation and on the community structure of mycorrhizal fungi. Chapter four describes experiments in which I studied the combined effects of insect root herbivory and mycorrhizal fungi on aboveground insect attack. In chapter five I investigated, whether plant-soil feedbacks affect mycorrhizal colonisation, plant chemistry and aboveground insect attack. Furthermore, I tested whether insect herbivore-induced changes. in soil microbial communities affect the performance of a new generation of plants and insect herbivores. The results from these experiments suggest (1) that insect shoot herbivores have less impact on arbuscular mycorrhizal fungi than insect root herbivores, (2) that the outcome of mycorrhiza-insect interactions largely depends on the plant species identity involved, (3) that insect-indu~d changes in non-mycorrhizal soil microbial communities might be an important mechanism eA'Plaining the productivity and composition of plant communities as well as the abundance of insect herbivores and (4) that interactions between mycorrhizal fungi and insect herbivores might sometimes be of low ecological relevance.