We will reveal the connection between surface structure and electronic properties of topological insulators by means of scanning probe microscopy and spectroscopy in combination with theoretical work based on density functional theory. We plan in particular to study the response of topological insulators to magnetic perturbations on the surface induced by evaporating iron atoms and forming small iron clusters by atomic manipulation. The interpretation of the recorded STM and AFM images will be supported by corresponding calculations. Accounting for spin-orbit coupling ensures in particular a proper description of topological surface states as well as of the magnetic configuration of the adatoms or deposited clusters. In parallel to experiment, charge and spin transport properties will be calculated in addition.