The increasingly manifest impacts of global warming have made it a global priority to phase out the use of petrol and diesel as transportation fuels in favour of hydrogen. But the production of hydrogen by most existing technologies entails substantial use of fossil fuels and CO2 emissions; indeed as much as 2.5–5tonnes of carbon is released as CO2 per tonne of hydrogen currently produced by conventional means. Hence the production of hydrogen can be ‘carbon-free’ only if it is generated by employing genuinely carbon-free renewable energy sources. The present review deals with the options, prospects, and challenges associated with this very high-priority area of global concern.