This title may at first disturb the disciplined scientific mind because of its apparent dimensional inconsistency. As a matter of fact, for our present purposes, it might just as well be titled "The Energy of Power." The point to be made, in either case, is that energy, as a concept, is tremendously powerful, both in the solution of technical problems and in its implications with reference to the true understanding of nature and the universe. And this is true whether we are speaking technically of energy or its time-derivative, power. Neither is an actual physical substance, of course, but each is an extremely useful and significant concept, without which the great contributions of modern science could hardly have been possible.