The aim of the project was to improve the estimation of pressure dependent velocity in sedimentary rocks, taking into account lithology (e.g. clay content) and pressure conditions. The project included theoretical and phenomenological analyses of intrinsic and stress induced seismic anisotropy. The anisotropic piezosensitivity approach (also called porosity deformation approach) was applied and developedfurtherto estimate and understand the stress dependence of seismic velocity in different anisotropic sedimentary rocks. Special emphasis was laid upon the reduction of free parameters for modelling and the physical interpretation of effective porosity and anellipticity as well as the change of symmetry of anisotropic rocks due to axial loading. The verification and adjustment of the model was supported by literature data and new laboratory measurements considering various sedimentary rocks ranging from pure sandstones over siltstones to shale. The research results contribute to a better theoretical understanding of 4D reflection seismic data with respect to anisotropy and in-situ stress conditions and enable the development of better velocity models for seismic imaging. It will also be possible to derive the stress conditions from the measured velocity anisotropy in the future.