This chapter on fuel cells covers the following topics: (a) fundamental electrochemical aspects and performance analysis; (b) technology research and development and demonstrations of fuel cell power sources for power generation, transportation, portable power, and space applications; (c) the role of fuel cells vs competing technologies, and (d) prospects for the applications and commercialization of fuel cell technologies in the twenty-first century. Although the fuel cell was invented in the nineteenth century, the twentieth century has been the period for technology development rather than widespread use. The fuel cell faces a great deal of competition in the proposed applications of power generation, transportation, and portable power. Significant work is still necessary, but intensified research and development activities could lead to the dawn of fuel cell commercialization and widespread use in the early part of the twenty-first century.