Abstract: A basic understanding of the relationship between structure and reactivity is needed for a targeted catalyst development scheme. Due to the complexity of real heterogeneous catalysts it is imperative to simplify the problem in a way such that an essential part of the complexity is included, while allowing one to investigate the catalyst system at the atomic level. Two models have been used, one is a metal single crystal in ultra-high vacuum conditions, and the other is metal clusters supported on planar oxide films under the same and also ambient conditions.