Fungal entomopathogens like Beauveria bassiana (Bals.) Vuill. (Ascomycota: Hypocreales) are known as antagonists of insects with multiple functional and ecological roles and have attracted increased attention as biocontrol agents in integrated pest management programs. Besides its entomopathogenic habit, evidence has accumulated that the fungus can also establish as an endophyte in a wide array of plant species. However, only limited information is currently available on the endophytic colonization of grapevine, Vitis vinifera (L.), plants with B. bassiana. In addition, the functional role of the fungus in planta, and/or the plant’s response to colonization by B. bassiana as well as the mechanisms underlying these responses and putative protection effects, still require elucidation. Therefore, the objectives of this thesis were to investigate whether the fungus B. bassiana is able to colonize grapevine plants, still maintains its entomopathogenic potential against insect pests, and can provide additional protection against plant fungal pathogens or limit their damaging effects. The investigation focused on the interaction between B. bassiana, grapevine plants, and potential target insect pests as well as fungal pathogens to gain more knowledge on this particular tritrophic interaction with regard to potential biological control strategies.