In the past decade, the aprotic lithium-oxygen(Li-O2) battery has generated a great deal of interest because theoretically it can store more energy than today’s lithium-ion batteries. Although considerable research efforts have been devoted to the R&D of this potentially disruptive technology, many scientific and engineering obstacles still remain to be addressed before a practical device could be realized. In this review, we summarize recent advances in the fundamental understanding of the O2 electrochemistry in Li-O2 batteries, including the O2 reduction to Li2O2 on discharge and the reverse Li2 O2 oxidation on recharge and factors that exert strong influences on the redox of O2/Li2O2. In addition,challenges and perspectives are also provided for the future study of Li—O2 batteries.