Hydrogen fuel-cell vehicles, largely forgotten as attention turned to biofuels and batteries, are staging a comeback. Conceptually, at least, a fuel cell is simply a device that takes oxygen from the air and hydrogen from a tank, and reacts them in a controlled way to produce water vapour and electric power. In a vehicle, that power can be directed through an ordinary electric motor to turn the wheels. In practice, fuel cell technology is anything but simple: Hydrogen must be derived from carbon-free renewable sources before fuel-cell vehicles can make a dent in the climate problem. One idea is to make the hydrogen by splitting water using electricity from wind farms or solar panels. This hydrogen must be distributed via special pipelines and tankers to an extensive network of hydrogen refuelling stations. But who will buy the vehicles if the stations don't exist? And who will invest in the stations if the vehicles dont' exist? Fuel-cell vehicles must store enough hydrogen to go several hundred kilometres between refuelling stops. Liquid hydrogen requires insulated tanks at -253C. So most companies have chosen to compress the hydrogen inside high-strength carbon-fibre tanks. The device converts hydrogen to electric power. The challenge is to make it light, cheap, robust and durable yet powerful enough to run the engine, lights and air conditioning. Even if car manufactures do get their fuel-cell vehicles to market by 2015, it will take years to establish a customer base, increse production and bring down costs. Meanwhile they are also pushing biofuels and battery-powered electric cars, each of which would require its own distribution system. Building these transportation infrastructures simultaneously might not be possible.