Back in the 1960s, Einstein's theory of general relativity re-emerged as a field of important research activity. Much of the impetus behind this resurgence came from powerful new mathematical ideas that Roger Penrose and Stephen Hawking applied to prove general singularity theorems for global space-time structures. Their results stirred the imaginations of astrophysicists and gave relativistic cosmology an entirely new research agenda. A decade later, black holes and the big hang model were on the tongues of nearly everyone who followed recent trends in science. As popular expositions dealing with quasars, pulsars, and the geometry of black holes began to appear in magazines and textbooks, Stephen Hawking reached a wide audience in 1988 with a lucid little book called A Brief History of Time. Twenty years later, it has emerged as one of the greatest scientific best-sellers of all time with some 10 million copies in print.