The 3D in situ stress field and its changes in geothermal reservoirs Information on the contemporary in situ stress state of the earth’s crust is essential for geotechnical applications and physics based seismic hazard assessment. Yet stress data records for a data point are incomplete and their availability is usually not dense enough to allow conclusive statements. This demands a thorough examination of the in situ stress field which is achieved by 3D geomechanicalnumerical models. However the models spatial resolution is limited and the resulting local stress state is subject to large uncertainties that confine the significance of the findings. In addition temporal variations of the in situ stress field are naturally or anthropogenically induced. In my thesis I address these challenges in three manuscripts that investigate (1) the current crustal stress field orientation (2) the 3D geomechanical numerical modelling of the in situ stress state and (3) the phenomenon of injection induced temporal stress tensor rotations. In the first manuscript I present the first comprehensive stress data ...