Gold is a chemically stable metal, however the gold nano-clusters (2−3 nm) supported on metal oxides, such as titanium dioxie, show high catalytic activity for CO oxidation at low temperatures. The origin of the catalytic activity is still in debate. So far, there are several hypotheses about the reactions on Au catalysts to explain the generation of the catalytic activity: 1. reaction at Au-TiO₂ perimeter, 2. reaction on Au atoms of Au nano particles (low-coordination Au atoms, negative charging Au atoms, Au atoms having specific electronic structure). In this review, we summarize recent investigations relating the hypotheses on Au/TiO₂ model catalysts. Also, we state several problems and issues for the future investigations.