Since the publication of A Brief History of Time - an introduction to some of the most important theories on the cosmos - Stephen Hawking is known widely as a man who has changed the way we look at the universe. He became a member of the Royal Society at the unusually early age of 32 and became in 1980 Lucasian Professor of Mathematics at Cambridge University. Written with Hawking's assistance, this is a readable account for the non-specialist of his contribution to theoretical physics. It encompasses his theories on the Big Bang and the nature of black holes, and contains the transcript of one of his most important essays.