On the face of it they are the answer to our prayers. Biofuels, their proponents say, would allow us to use our cars every day while the plants grown to produce the fuel would effectively suck up the CO2 thrown out of the exhaust pipe.
A kind of perfect cycle, but one that not everyone finds convincing.
Look back in history and you will find that pioneers of motor technology, like Rudolph Diesel, devised their engines to run on biofuels.
The low cost of fossil fuel production meant that greener alternatives were sidelined.
But in recent years there has been dramatic growth in production of bio-ethanol and bio-diesel.
Suger cane, sugar beet, wheat, maize and potatoes can all be used to produce sugars that are fermented into ethanol, and then mixed with petrol.
Oil from crops liks rape and sunflower can be used to make biodiesel, which can then be used as it is, or blended.
The rise in the price of fossil fuels, security concerns over oil producing countries and environmental issues have all boosted the production of biofuels.
They are also promoted by some vehicle manufactureres because they increase engine life.
In terms of production a clear division can be seen.
Europe is the world leader in biodiesel, Germany ahead of France and Italy. Bioethanol is the domain of the Americas, with Brazil far ahead.
Nevertheless, biofuels only account for a tiny percentage of the fuel used in transport worldwide.
And they may stay that way if some in the green lobby win their argument.
They say biofuels threaten biodiversity and food security, and the energy used producing them cancels out a sizeable chunk of the CO2 emission benefit they offer.
Brazilian environmentalist Carlos Young argues that the benefit of biofuel use in his country is far outweighed by the rate of deforestation.
Those who back biofuel argue that second generation technology under development will mean deforestation to grow easily refined crops like sugar cane will become less of an issue.