Recent material developments of fast solid oxide and lithium ion conductors are reviewed. Special emphasis is placed on the correlation between the composition, structure, and electrical transport properties of perovskitetype, perovskite-related, and other inorganic crystalline materials in terms of the required functional properties for practical applications, such as fuel or hydrolysis cells and batteries. The discussed materials include Sr-and Mgdoped LaGaO 3 , Ba 2 In 2 O 5 , Bi 4 V 2 O 11 , RE-doped CeO 2 , (Li,La,)TiO 3 , Li 3 La 3 La 3 Nb 2 O 12 (M=Nb, Ta), and Na super-ionic conductor-type phosphate. Critical problems with regard to the development of practically useful devices are discussed.