Although gold (Au) in bulk is almost inert as a catalyst, it turns to be active for many reactions when deposited as nanoparticles (NPs) and clusters on base metal oxides. They are characterized by three features; active at low temperatures, enhanced by water, and uniquely selective. Over supported Au NPs CO adsorbs on the Au surfaces and O₂ is activated at the perimeter interfaces around Au NPs. Both the two adsorbates react with each other at the perimeter interfaces. Gold clusters are expected to exhibit unique selectivity through the precise control of the number of atoms and the selection of appropriate support materials.