Fuel cells are a proven power source for manned spacecraft where they generate electrical power from stored hydrogen and oxygen that are carried in cryogenic liquid form. The world?s petroleum production is about to pass its peak, so a world wide development effort is being directed into adapting these high efficiency fuel cells into powering automobiles, buses and trucks. Fuel cells are more efficient than the secondary batteries used. First Alkali fuel cells are used in aircrafts, but now a days fuel cells using Proton Exchange Membrane are used. A fuel cell can theoretically, deliver 500 kilowatt-hours per kilogram of hydrogen plus oxygen. Today?s best lithium batteries can deliver around 120 kWh per kilogram. Fuel cells can convert fuel to electric power with an efficiency of over 80%. Even a diesel engine cannot do better than 40% at it?s optimum speed and load. In the past, fuel cells contained a platinum catalyst that is very costly. Now a nickel-tin catalyst which works was discovered. As a result, fuel cells can be a possible substitute for batteries in spacecraft. The different power requirements in an aircraft & the application of fuel cells on aerospace are discussed in this paper. The fuel cell will find applications that lie beyond the reach of the internal combustion engine. Once low cost manufacturing is feasible, this power source will transform the world and bring great wealth potential to those who invest in this technology. It is said that the fuel cell is as revolutionary in transforming our technology as the microprocessor has been. Once fuel cell technology has matured and is in common use, our quality of life will improve and environmental degradation caused by burning fossil fuels will be decreased. It is generally known that the maturing process of the fuel cell will not be as rapid as that of microelectronics.