The adsorption properties and reactivities of gold are summarized in terms of their size dependency from bulk to fine particles, clusters and atoms. The catalytic performances of gold markedly depend on dispersion, supports, and preparation methods. When gold is deposited on select metal oxides as hemispherical ultra-fine particles with diameters smaller than 5 nn, it exhibits surprisingly high activities and/or selectivities in the combustion of CO and saturated hydrocarbons, the oxidation-decomposition of amines and organic halogenated compounds, the partial oxidation of hydrocarbons, the hydrogenation of carbon oxides, unsaturated carbonyl compounds, alkynes and alkadienes, and the reduction of nitrogen oxides. The unique catalytic nature of supported gold can be explained by assuming that the gold-metal oxide perimeter interface acts as a site for activating at least one of the reactants, for example, oxygen. Some examples and future prospects in applications are also briefly described.