Thirty years ago Albert Einstein died, his dream of a theory that would unify the universe unfulfilled. He spent the last decades of his life searching for such a theory-a theory that would explain everything from elementary particles and their interac- tions to the overall structure of the universe. But he failed, t because he didn't try hard eugh, but because the attempt was ahead of its time. When Einstein worked on the problem liter- ally thing was kwn about black holes, white holes, sin- gularities, the Big Bang explosion and the early universe, quarks, gauge invariance, and weak and strong nuclear forces. Today we kw that all these things are important in relation to a unified theory, and that they must be incorporated in and explained by such a theory. Thus, in a sense, our problem is much more complex today than it was in Einstein's day. But scientists have persevered and as a result we are w tan- talizingly close to achieving this long-sought goal. Important breakthroughs have been made. In this book we will look at these breakthroughs and at recent unified theories-theories that go by the names supergravity, superstrings, GUTs, and twistor theory. In order to understand the problem, however, we must begin at the beginning.