The recent success of fuel-cell-powered demonstration vehicles using the proton exchange membrane fuel cell developed by the Canadian company Ballard Power Systems, by DaimlerChrysler and many others, suggests that fuel cells have finally come of age. Elsewhere, Plug Power and a number of other developers are striving to bring their domestic grid-independent power supplies onto the market within the next couple of years. What has driven these developments? And what exactly is a fuel cell?