The behaviour of interface-dominated metal oxide thin films is largely influenced by nanoionic effects. The control and engineering of interfaces becomes a powerful tool for the design of optimized electrochemical devices, in which ion movement plays a primary role. This chapter provides a comprehensive description of the most relevant ionic phenomena taking place in interfaces, as the basis of nanoionics. Grain boundaries and strain-dominated thin films are analysed in detail, highlighting the fascinating relation between structure and chemical properties. The main strategies for an active implementation of these two interfaces in real devices are discussed. The chapter concludes overviewing foreseen future applications of nanoionics in superior energy and information technologies based on metal oxide thin films.