A combined theory of k.p-perturbation theory and the semiconductor Bloch equations (SBE) is used to simulate photo currents in GaAs based systems. The focus lies on the so-called shift current, a microscopic current caused by the spatial motion of excited carriers inside the crystal structure. The validity of the combined theory is tested for the example of bulk GaAs using known symmetry properties of shift currents. Using the SBE, which allow for a non-perturbative and k-resolved analysis of shift currents, various linear and non-linear properties of shift currents are investigated, in particular, signatures of Rabi-oscillations in bulk and the influence of band-mixing in quantum well systems.Excitonic effects in shift currents are investigated for a full three-dimensional band structure. The inclusion of Coulomb interaction is numerically demanding and normally done using approximations, e.g., a parabolic band structure. Such approximations cannot be applied for shift currents.To deal with this numerical challenge, the development of a new non-uniform grid is necessary. The convergence and accuracy of the new grid as well as the obtained results for the exciton binding energy and the shift current are presented and discussed.A novel method is developed which consists of combining k.p-perturbation theory with real-space wave function obtained from density functional theory.The method allows to simulate shift currents in real space with atomic resolution.