Rechargeable metal-oxygen batteries are receiving significant interest as a possible alternative to current state of the art lithium ion batteries due to their potential to provide higher gravimetric energies, giving significantly lighter or longer lasting batteries. Recent advances suggest that the Na-O2 battery, in many ways analogous to Li-O2 yet based on the reversible formation of sodium superoxide (NaO2), has many advantages such as a low charge overpotential (~ 100 mV) resulting in improved efficiency. In this perspective we discuss the current state of knowledge in Na-O2 battery technology, with an emphasis on the latest experimental studies, as well as theoretical models. We offer special focus on the principle outstanding challenges and issues, and address the advantages / disadvantages of the technology when compared with Li-O2 batteries as well as other state-of-the-art battery technologies. We finish by detailing the direction required to make Na-O2 batteries both commercially and technologically viable.