With the increasing importance of electrified transport, the need for high energy density storage is also increasing. Possible candidates include Li-O2 batteries, which are the subject of rapidly increasing focus worldwide despite being in their infancy of understanding. This excitement owes to the high energy density of Li-O2 (up to 2-3 kWh kg−1), theoretically much higher compared to that of other rechargeable systems, and the open “semi-fuel” cell battery configuration that uses oxygen as the positive electrode material. To bring Li-O2 batteries closer to reality as viable energy storage devices, and to attain suitable power delivery, understanding of the underlying chemistry is essential. Several concepts have been proposed in the last year to account for the function and target future design of Li-O2 batteries and these are reviewed. An overview is given of the efforts to understand oxygen reduction/evolution and capacity limitations in these systems, and of electrode and electrolyte materials that are suitable for non-aqueous and hybrid (nonaqueous/aqueous) cells.