The major theme of this book is the effect of Au particle size on the thermal catalytic activity, photocatalytic activity, and electrocatalytic activity of Au nanoparticle-loaded metal oxides. Chapter 1 presents the unique and interesting physicochemical properties of Au nanoparticles and Au nanoparticle-loaded metal oxides in connection with the following topics of thermal catalysis, photocatalysis, and electrocatalysis. Chapter 2 describes two techniques for forming Au nanoparticles on the metal oxide surfaces with their particle size controlled. One is the heating temperature-varied deposition-precipitation method. The other is the deposition precipitation-electrochemical reduction method. Chapter 3 concerns the Au particle size-dependence of the thermal catalytic activity of Au/TiO2 for hydrogen peroxide decomposition. Chapter 4 deals with the Au particle size-dependence of the photocatalytic activities of Au/TiO2 for the reductions of nitrobenzene to aniline and 2,2’-dipyridyl disulfide to 2-mercaptopyridine. Chapter 5 states the size-dependence of Au nanoparticles on the electrocatalytic activity in CdS quantum dot sensitized solar cell.