Application of scanning tunneling microscopy (STM) and other surface physical techniques to processes on well-defined single crystal surfaces enables investigations of the atomic scale steps involved in heterogeneous catalysis. Examples presented include effects caused by the operation of interactions between adsorbed particles and the role of surface defects as I active centers'. The most detailed information is nowadays available for the CO oxidation at a Pt(1 1 1) surface. Finally, the transformation from metal to oxide will be identified as the source for the apparent 'pressure gap' in carbon monoxide oxidation on ruthenium, whereby also the concept of coordinatively unsaturated sites (cus) will be elucidated. (C) 2002 Elsevier Science B.V. All rights reserved.