Electronic and optical properties of semiconducting materials, devices and nanostructures are governed by defects and interfaces that introduce electronic states into the bandgap. In order to address the relation of their atomic and electronic structure, atomically resolved transmission electron microscopy (HRTEM) is combined with spectroscopic techniques. Currently, our research is mainly focused on semiconductors for photovoltaic applications aiming at understanding and quantitatively modeling defect reactions at the atomic level as a basis for successful defect engineering in such materials.