Although gold in bulk is poorly active as a catalyst, it exhibits surprisingly high catalytic activity when deposited as nanoparticles (NPs) on base metal oxides. The catalytic performance of supported gold NPs can be created by choosing the kind of support materials, by controlling the size of gold, and by building up strong contact of gold with the supports. Since perimeter interfaces around gold NPs act, in principle, as the active sites for oxidation and hydrogenation, gold should be smaller than 10 nm in diameter. A new area of research is clusters, which are smaller than 2 nm in diameter and less than 200 atoms. Gold clusters possess electronic structures different from those of bulk gold and at the same time provide increased fractions of edges and corners which are highly unsaturatedly coordinated sites. Accordingly, gold clusters will be blessed by unique catalytic performance, and many examples have recently been emerging. This article summarizes such examples in terms of "size- and structure-specificity," covering gas-phase free clusters, polymer- or organic-ligand-stabilized clusters in liquid phase, and clusters supported on base metal oxides, carbon materials, and organic polymers for gas-phase and liquid-phase reactions.