Progress in the development of oxygen ion and mixed conductors is responsible for innovations in gas sensors, fuel cells, oxygen permeation membranes, oxygen pumps and electrolyzers. Commercialization has been impeded by material stability and compatibility issues, high fabrication costs and an inadequate understanding of the interfacial phenomena controlling the operation of the devices. Here, a group of experts cover all the key topical areas, ranging from fundamentals relating to (a) defects, electrochemical and interfacial processes, (b) catalysis, electrocatalysis and gas reforming, to design and fabrication, including (c) advanced electroceramic processing methods, (d) materials selection and optimization, and (e) applications including scale-up, commercialization and competitive technologies.