Fuel cells provide the direct conversion of chemical into electrical energy. Therefore they are regarded as an attractive concept for numerous technical applications. However, the success of fuel cell systems on the market is still limited as in many cases they are economically not competitive and their technology is not mature. For example, the smooth and efficient operation of PEM fuel cell systems requires a sophisticated water and heat management. The author presents models for the description of the coupled heat and mass transport processes in fuel cells together with results he achieved by computer simulation for various operating modes and conditions.