As a first practical application, fuel cells were used in the Gemini and Apollo space programs. In the field of transportation, fuel cells are probably the most serious contenders to compete with internal combustion engines (ICEs). They are highly efficient because they are electrochemical rather than thermal engines. Hence, they can help to reduce the consumption of primary energy and the emission of CO₂. What makes fuel cells most attractive for transport applications is the fact that they emit zero or ultralow emissions. Clearly, the automotive market is by far the largest potential market for fuel cells. When developersstarted doing their first cost calculations, they realized they were in for steep competition against improvedinternal combustion engines, hybrid cars, and other possible contenders.