The investigations performed in the frame of this study were on the one hand intended to support and guide the development of an alternative drilling technology for deep geothermal drilling and on the other hand to provide a significant impact on the understanding of the rock destruction process. During the development of the alternative drilling technology, in which high-pressure fluid jetting with in excess of 2500 bar jet pressure is combined with mechanical drilling methods, a comprehensive experimental study was conducted. The aim of this study was to investigate the cutting performance of the fluid jets in hard rock formations under various ambient pressure conditions. The experiments, performed under different ambient pressure regimes, show a completely different cutting performance than under atmospheric conditions. The main influencing parameters were determined and adapted to enable a sufficient performance for all tested conditions. Furthermore, the usage of drilling fluids in place of water was investigated. The study shows that high-pressure jetting is feasible in the challenging simulated downhole environment, including high ambient pressure, several jets with different pressures acting simultaneously, drilling mud as jetting resp. surrounding fluid and high traverse velocities. Full-scale drilling experiments (8½ in. bit size) with the jet-assisted rotary drilling system were performed in hard crystalline rock to validate the given data and to gain more detailed information about the drilling system performance under realistic conditions. A total number of seventeen test runs was conducted to enable a comparison of drilling performance between state-of-the-art drilling methods and the innovative jet-assisted system. A significant increase in the rate of penetration was achieved with the alternative drilling system. Another relevant aspect was the influence of different drilling and jetting fluids on the drilling performance. Detailed research on the interaction of the hydraulic and mechanical rock removing processes led to further conclusions about the rock destruction process. Lessons learned during these experiments were directly integrated in the preparation of later field tests. The alternative drilling system was tested in a 1.3 km deep wellbore under real drilling conditions. The field test was fully satisfying, finally proving the feasibility and efficiency of this technology. In order to further optimize the drilling technology, but also to generate a more thorough understanding of the underlying mechanisms, comprehensive research activities were conducted. Due to the complex nature of the rock destruction process, numerical simulations are regularly performed. Therefore, the determination of reliable input parameters is critical. While efficient guidelines and established procedures exist for compressive loading, the current state-of-the-art testing methods for tensile loading in the majority of the cases are inadequate. For that reason, an innovative purely mechanical test setup is proposed in this study, which eliminates the identified confounding effects. Furthermore, an alternative procedure for the investigation of rock dilatancy under unconfined conditions is presented. The proposed procedure is based on torsion tests because of their capability to generate stress states of pure shear without exhibiting axial splitting and related negative effects. With the help of numerical simulations, a reasonable range for the dilatancy angle can be estimated and compared to common methods of determination via compression tests. Thus, vital understanding of the different behavior of sedimentary and crystalline rocks due to various loading situations was revealed.