This book focuses on research related to ionic conducting (e.g., protons, oxygen ions) materials and devices. Contributions range from fundamental materials R&D, to characterization, to materials for batteries, sensors, membranes, supercapacitors and fuel cells. Special emphasis is given to miniaturized solid-oxide fuel cells (micro-SOFCs), from fundamental materials studies which are still very much needed for this application, to the development of devices. Innovative concepts for energy storage are also discussed.