The drive towards an electricity based economy and emerging green technologies has resulted in a tremendous push to create safer and more efficient energy storage devices.1–3 The development of solid-state batteries is a major effort in this direction4,5. Unlike the case of a traditional electrochemical apparatus, in solid-state batteries ions move through a solid crystalline electrolyte. Ionic motion is thus intimately linked to the condensed matter description of the system – that is, the periodic electronic and ionic properties of the crystal - and is not adequately described by the existing electrochemical tenet. In the present article, we propose a microscopic, first-principles, description of the ionic conduction in crystals. This allows us to understand the ideal characteristics of materials for ionic conduction in general, and for solid-electrolyte applications in particular. Using ab initio calculations, we show that our formalism results in ionic mobilities consistent with experiments for several materials. Our work opens the possibility for the development of solid electrolytes based on fundamental physical principles rather than empirical descriptions of the underlying processes.