This book has introduced the development of metal–air batteries in the order of Li–air battery, sodium–air battery, zinc–air battery,Mg–O2 battery, and flexible metal-air batteries, with the focus on the Li–air battery. In details, various topics, which include metallic anode, discharge products, electrolyte, parasitic reactions,electrocatalysts, mediator, and spatiotemporal operando X-ray diffraction study in Li–air battery, are described separately in each chapter. Simultaneously, the contents of in situ spectroelectrochemical techniques developed inmetal-air batteries and novel methodologies to model charge transport in metal-air batteries are also discussed separately in each chapter.With these information, our understanding on the reaction processes taking place in these metal-air batteries is expected to be deepened. The contents of this book are aimed at professionals at all levels in the field of metal–air batteries, providing them with a quick access to the development of metal–air batteries. The state of development in the metal–air batteries has informed us that this was an appropriate time for this book. Even though significant progress has been made, much still remainsto be done to explore the science within these metal–air batteries. As a benefit, convincing guidance can be provided toward the final practical application of these technologies. Finally, we express sincere gratitude to all the contributors to this book who are actively engaged in research at the cutting edge of metal–air batteries.