A hydrogen vehicle is an alternative fuel vehicle that uses hydrogen as its onboard fuel for motive power. The power plants of hydrogen vehicles convert the chemical energy of hydrogen to mechanical energy either by burning hydrogen in an internal combustion engine, or by reacting hydrogen with oxygen in a fuel cell to run electric motors. Hydrogen fuel does not occur naturally on Earth, and thus is not an energy source, but is an energy carrier. The incentives for a hydrogen economy are the emissions, the potentially CO2-free use, the sustainability and the energy security which shows that hydrogen has excellent prospects to achieve very satisfactory performance in engine applications that may be superior in many aspects to those with conventional fuels. Integrated wind-to-hydrogen plants, using electrolysis of water, are exploring technologies to deliver cost low enough, and quantities great enough, to compete with traditional energy sources. The question whether hydrogen can be manufactured abundantly and economically will remain the limiting factor to its widespread use as an S.I. engine fuel in the future.