Renewable energy sources have become significant sources of power in recent years due to their abundance in nature. Solar, wind, hydro, tidal, and biomass are the various renewable energy sources that serve as pollution free sources compared to other conventional sources of power such as coal, oil, etc. One of such environment friendly technique is fuel cell system which generates power almost negligible emission. This chapter offers a thorough discussion on fuel cells, encompassing their basic principle, thermodynamics, their components and performance characteristics. It gives a comprehensive insight on different existing fuel cells examining the type of electrolyte used, operating temperature, efficiency and their unique characteristics. The application of the discussed fuel cells are highlighted in this chapter.