His work reflects a concern with both the theoretical and the experimental aspects of X-ray spectroscopy, including both fundamental studies and applications. His current interest is in the use of X-ray spectroscopies for the study of the electronic and magnetic structure of condensed matter, in particular for transition metal oxides, nanoparticles and heterogeneous catalysts under working conditions. He has published 320 papers. (22500 citations, h-index 74; cf. google scholar).