For transportation applications, hydrogen is both a combustion fuel and a fuel cell reactant. It contains no carbon, which means no carbon dioxide emissions when combined with oxygen in air to produce useful energy and water. It shares the dual distinctions of having the highest mass energy density and the lowest volume energy density of any fuel at a given temperature or pressure. On an energy basis, it is the most difficult fuel to store onboard a vehicle. It is the only fuel acceptable for direct use as a fuel cell reactant, and in this capacity is currently used by all fuel-cell electric vehicles. If produced by electrolysis of water using electricity solely generated from zero-carbon sources (solar or wind), it is referred to as green hydrogen. But over 95% of all hydrogen is currently produced from natural gas or coal, solely for the production of ammonia for agriculture. From these sources, the production process is highly carbon intensive. In this chapter, we cover all aspects of hydrogen as a vehicle fuel or energy carrier related to transportation. The long history of hydrogen as a synthetic combustion fuel is reviewed. The technologies of both hydrogen IC engines and hydrogen fuel cells are presented and discussed. The environmental impact of its overall production and use is examined.