The development of dense ceramic membranes for separating and reacting various gaseous components has been under intense investigation during the past several years. Such applications include the separation of oxygen from air, the partial oxidation of methane and other hydrocarbons, and the separation of hydrogen from synthesis gas or other hydrogen-containing process streams. The US Department of Energy is currently sponsoring major efforts in the development of these membranes for advanced power and fuels production applications. This article provides an overview of the current programme. Particular emphasis is placed on the separation of oxygen from air.