Hydrogen (H2) is the lightest and simplest of all gases. It can be manufactured from water (H2O) by the input of electricity derived from clean energy sources. When burned in air, hydrogen reverts back to pure water. Because of this simple, clean energy cycle, hydrogen has been called the “fuel of the future,” and it is possible to formulate a “hydrogen energy economy” that is completely free of carbonaceous fossil fuels and their associated contributions to global warming. This lecture will cover the concept and history of hydrogen energy in straightforward terms. We shall also discuss the principal economic and infrastructure challenges that must be overcome before hydrogen energy can fully displace conventional fuels derived from coal, petroleum and natural gas. One of the early commercial applications of hydrogen energy will be fuel cell powered transportation, with California leading much of the effort.