The work presented here would be impossible without the engaged efforts of the co-workers of the group. Results from W. Ranke, Ch. Kuhrs, G. Ketteler, T. Ressler, J. Wienold, F. Girgsdies, R. Jentoft, H. Bluhm, A. Knop-Gericke, M. Haevecker and E. Kleimenov were used for illustration. The author is indebted to them and to their respective co-workers and to all contributing to the design and building of the necessary equipment. The fruitful co-operation with M. Salmeron and D.F. Ogletre was mentioned. Support is acknowledged from the synchrotron radiation facilities BESSY II, HASYLAB and ESRF, from the Deutsche Forschungsgemeinschaft (SPP 1051), from the Fonds der Chemischen Industrie and from the ATHENA co-operation.