We review the production, distribution, storage, combustion, environmental impacts, safety, regulation, and lifecycle costs of hydrogen as a fuel for motor vehicles. We find that: (1) steam reformation of natural gas is the most economical means of producing hydrogen, but renewable sources may become cost competitive early in the next century; (2) given the difficulties in constructing a dedicated hydrogen infrastructure, small-scale, decentralized reformation along existing natural gas lines would be the preferable means of distribution for the near-term; (3) of the many ways to store hydrogen onboard a vehicle, no method (yet) is inexpensive, lightweight, and compact; (4) hydrogen engines produce an order of magnitude less hydrocarbons, carbon monoxide, greenhouse gases, and toxic air pollutants than do controlled gasoline engines; (5) hydrogen's dangers are different from but not necessarily worst than those of gasoline; and (6) hydrogen combustion vehicles will be more expensive on a lifecycle basis than conventional vehicles, but hydrogen fuel cell vehicles could be cost competitive due to high efficiencies and long component lifetimes.