The general principle of the conversion of sunlight into chemical energy or electricity consists of the generation of electron-hole pairs by the absorption of incident light and their subsequent spatial separation. Charge carriers are consumed by an electrochemical reaction (photoelectrochemical cell) or in an external circuit (photovoltaic cell). In the particular case of photoelectrochemical conversion, electrochemical reactions such as hydrogen evolution and CO2-reduction are mostly catalyzed by metals and alloys. Thus, this book will usually refer to the metal phase to signify a catalyst surface.