Ionics in solid state electrochemistry is inherently connected with the chemistry of defects in solids, and this chapter provides some elementary considerations. Ion conductivity or diffusion in oxides can only take place because of the presence of imperfections or defects in the lattice. Nonstoichiometry occurs when there is an excess of one type of defect relative to that at the stoichiometric composition. Electronic defects may arise as a consequence of the transition of electrons from normally filled energy levels, usually the valence band, to normally empty levels, the conduction band. As in aqueous electrochemistry, research interest in the field of solid state electrochemistry can be split into two main subjects: Ionics: in which the properties of electrolytes have the central attention Electrodics: in which the reactions at electrodes are considered. The chapter also presents an overview of the key concepts discussed in this book.