This thesis covers the theoretical discussion of defects, which are crucial for the properties of modern materials. The computational method of choice was the Green’s function method (GF), which performed very well in the description of the magnetic or electronic properties of single adatoms, point defects or disordered materials. In particular, the underlying multiple scattering theory allowed the efficient self-consistent calculation of the considerably differing characteristics of these systems. The crystal field parameters for a single Holmium adatom on top of Pt(111) were calculated. The threefold symmetry of the system mostly causes the long lifetime observed in experiments. The effect of oxygen vacancies and antisite defects onto the magnetic properties of SrCoO3-δ and Sr2FeMoO6-δ were intensively studied. For the investigation of short-range order effects, the impact of different local order regimes onto the electronic and equilibrium properties was studied for AgcPd1-c alloys.