Fuel cells are being enthusiastically discussed as a new, efficient and clean power supply for a wide variety of applications from electronics to electric vehicles to decentralized power generation. To help clarify the discussion, the fundamentals of fuel cell structures and operation will be presented. The types of fuel cells and their characteristics will be discussed, along with examples of applications. Proton exchange membrane (PEM) fuel cells have been used in the space program beginning in the 1960’s. Alkaline electrolyte fuel cells are currently used in the Space Shuttle. There are a few hundred fuel cell systems already in use in several countries, operating very reliably. Many fuel cell powered vehicles have been demonstrated, including buses, autos, golf carts, and boats. Economics is a main issue that needs to be better addressed before fuel cells are ready for most applications. With regard to consumer fuel cells, small power units for electronic applications will be available first.