In the last few decades, there are some exciting developments in the field of lithium (Li)-ion batteries from small portable devices to large power system such as electric vehicles (EVs). However, the maximum energy density of lithium-ion batteries is insufficient for the extended range of EVs propulsion. On the other hand, metal-air batteries have a greater power storage capacity, a few times more than the best performing lithium-ion batteries. Mechanically rechargeable zinc (Zn)-, magnesium (Mg)-, and aluminum (Al)-air batteries are receiving increasing attention, due to the advantages of using safe, low cost and abundant materials. If successfully developed, these batteries could provide an energy source for EVs comparing that of gasoline in terms of usable energy density. Nevertheless, there are still numerous scientific and technical challenges that must be overcome, if this alluring promise can be turned into reality. This paper provides a comprehensive overview of recent advances and challenges of metal air batteries from various elements, including air cathode, electrolyte, and anode. In addition, this review outlines the fundamental principles and understanding of the electrochemical reactions in the areas of lithium-air batteries. Finally, a summary of future research directions in the field of the metal-air batteries is provided. (C) 2013 The Electrochemical Society. All rights reserved.