Microorganisms are ubiquitously present. They are not silent witnesses of the life of higher organisms instead they may have a profound impact in their ecological success or failure. Microorganisms can improve or reduce their host fitness acting as symbionts or as pathogens. In some cases they are essential for the survival of their host as exemplified by obligatory symbionts of phloem feeding insects. In any case, they play a fundamental role in the outcome of plant-insect interactions, they are determinant for the survival of the plant and more importantly our crops. In spite of their important influence, microorganisms tend to be forgotten when studying the ecology and evolution of tri-trophic interactions. Microorganisms might affect several aspects of plant-insect and insect-parasitoid interactions: For example, host plant selection by enabling or disabling insects to colonize a particular plant; improvement of the plant nutritional status and immune defenses; changes in the phloem composition modifying the attractiveness of the host plant; and changes in insect resistance to parasitism. These complex interactions are bound to be disrupted by climate change. Understanding them is a challenge we must take up to improve our plant protection procedures. We have entered the post-genomic era: Now several microorganism, plant and insect genomes are available. Advances in sequencing technologies are revolutionizing transcriptome and proteome analyses. The conditions have never been better to start unraveling the interdependency of microorganisms with their associated plant or animal partners. The present session proposes to discuss the role of microorganisms as motors driving tri-trophic interactions. This session will be composed of a series of oral presentations on the role of microorganisms in the outcome of tri-trophic interactions. Talks will present the impact of insect symbionts on plant-insect interactions and on insect-parasitoid interactions, finally the role of plant symbionts will be addressed.