Publisher Summary This chapter discusses the metals and the characteristics of zinc-air primary cells, zinc-air secondary batteries, and iron-air secondary batteries. In relation to their physical size, zinc-air batteries store more energy per unit of weight than any other primary type. The zinc-air system is capable of operating down to -18°C and giving 80% of the 21°C performance at this lower temperature. Zinc-air secondary rechargeable batteries are still in development phase. Experimental cells have given 155-175Wh&g at the C/5 rate of discharge. These energy densities are approximately twice those of many of the best existing rechargeable systems. Further, cadmium-air secondary batteries are also in development phase. The cadmium-air system has a theoretical energy density of 445 W h&g. The cadmium anode used is the type that has demonstrated good stability and low self-discharge in other alkaline systems, such as the nickel-cadmium battery. In addition to this aluminium-air secondary batteries is discussed.