Au-based catalysts have great potential because of their unique activity and selectivity for a variety of important reactions, The special catalytic properties of supported An nano-particles depend critically upon the particle morphology, i.e. size, shape and thickness, as well as support effects. This paper reviews the current understanding of CO oxidation on supported Au catalysts. The electronic structure of Au particles at various nucleation sites and on different supports is summarized, and the effect these changes have on catalytic performance is discussed, Recent results from our laboratories have demonstrated the synthesis of well-ordered Au mono- and bi-layer films on a titanium oxide support and show that the active Au structure for CO oxidation is an electron-rich, ALL bi-layer. In contrast, the monolayer structure, which may involve the TiO4 support, is significantly less active (by less than an order of magnitude) than the Au bi-layer. The oxidation state of the Au and how this relates to the catalytic activity are also discussed. (c) 2005 Elsevier B.V. All rights reserved.