Throughout most of human history, humankind has relied on the so-called "renewable" energy resources--wood, plants, animals, wind, and falling water--for its energy needs. In the nineteenth century, a major transition occurred to reliance on a fossil energy source: coal. The twentieth century has seen a second major energy transition from coal to petroleum and natural gas. Now another important energy system transition has begun. Within the next hundred years, the United States and other industrialized nations will have to make a new transition to reliance either on nuclear fission and fusion sources or, what now seems more likely, the traditional "renewable" resources.