Entomopathogenic fungi (EPF) are well-studied natural enemies of insect and mite pests and several isolates with relatively broad host ranges have been developed as biological control agents. Besides their direct interaction with insect and mite hosts as pathogens, these fungi are also able to associate symbiotically with plants as endophytes, hence living inside plant tissues asymptomatically. Several species of EPF can be experimentally established as endophytes in a range of plant species, including important crops, causing growth promotion and affecting plant-herbivore interactions. The effects of these endophytes on insect herbivores have been widely studied, but the mechanisms behind the reported effects are not documented. The general absence of fungal propagule production in colonized plant tissues and lack of infection in insects feeding on endophytically colonized plants support the notion of antibiosis and feeding deterrence over direct infection by EPF endophytes. Moreover, plants colonized by EPF below-ground can lead to effects on herbivores above-ground, suggesting complex interactions between the two organisms, potentially mediated by changes in the physiological response of the plant. This thesis addresses the question of how EPF seed inoculations can alter plant physiology with a focus on modifications of the activities of key carbohydrate and antioxidant enzymes and profiles of specific plant secondary metabolites (PSMs) and evaluate the potential role of these compounds in plant-fungal-herbivore tripartite interactions. The capacities of three isolates of EPF, belonging to the species Beauveria bassiana, Metarhizium brunneum and M. robertsii, respectively, to establish as endophytes were evaluated through seed inoculation of important crop plants of three different plant families (Poaceae, Fabaceae and Solanaceae). The inoculations were evaluated for the effects on plant growth parameters, the population growth of arthropod herbivores and different physiological parameters in leaves of the host plants were measured with and without herbivore attack. Overall, B. bassiana was re-isolated from all tested plant parts (leaves, stem and roots), M. brunneum from stem and roots and M. robertsii only from roots of all tested plant hosts. Compared to control treatments, inoculations with M. robertsii significantly reduced herbivore population growth, while populations on M. brunneum inoculated plants unexpectedly increased. This pattern was observed across all tested crops. Inoculations with B. bassiana were related to negative or neutral effects on the population growth of arthropod herbivores as compared to control plants. Overall, the two isolates of Metarhizium spp. increased plant growth as compared to uninoculated control. Three scientific manuscripts were prepared based on current researchIn Manuscript I, the activities of key carbohydrate and antioxidant enzymes were measur ed in wheat leaves after fungal inoculations of seeds and infestation by the aphid, Rhopalosiphum padi. Inoculations with M. robertsii significantly reduced, M. brunneum increased while B. bassiana had no effects on population growth of R. padi as compared to control plants. The activities of most ofthe selected carbohydrate enzymes increased in wheat inoculated with both isolates of Metarhizium spp. while showing no clear correlation with a population growth of aphids. Whereas, activities of antioxidant enzymes showed minimal changes by EPF treatments and as a response to aphid feeding. However, M. robertsii increased the activities of superoxide dismutase while M. brunneum increased the activities of catalase and glutathione S transferase, which likely played a role in aphid population growth. This indicated that specific antioxidant enzymes played a role in aphid population growth by fungal inoculations while carbohydrate enzymes are likely linked with plant growth. Manuscript II addresses the quantification of a range of PSMs in two separate host plants, belonging to benzoxazinoids in wheat and flavonoids in bean as response to EPF seed inoculation and aphid infestation to elucidate the role of these specific compounds in the plant-fungus-herbivore interactions. Wheat was infested with R. padi, while the aphid Aphis fabae was used to infest bean plants. Concentrations of more than half of the PSMs were affected by fungal and aphid treatments and the changes in aphid numbers were associated with this regulation rather than the endophytic colonization of above-ground plant tissues. Nor were PSMs related to growth promotion effects by EPF inoculations. The three fungal isolates produced comparable effects on aphid populations and PSMs across the two host plants. This study links for the first time the effects of EPF endophytes against aphids with plant physiological responses of wheat and bean. In Manuscript III, the variability of two selected steroidal glycoalkaloids was determined in tomato leaves by LC-MS after fungal seed inoculations and infestations by two-spotted spider mite, Tetranychus urticae, a herbivore possessing a different feeding style than aphids. Population growth of spider mites was highest on plants inoculated with the isolate of M. brunneum, which also showed relatively low concentrations of steroidal glycoalkaloids in the leaves compared to the other treatments. In contrast, tomato plants inoculated with the isolates of B. bassiana and M. robertsii produced significantly. Overall, the research reported in this thesis opens new insights into the effects of EPF endophytes on plant physiology to cause ecological effects against herbivores. The study provides a link betweenthe ecological effects and the physiological responses of plants caused by EPF inoculations which may have implications in plant protection strategies. The understanding of plant physiological changes by beneficial fungi are important for future applications of EPF for herbivore management