This book contains a slightly expanded version of the Clarendon Lectures in Law given in Oxford in 2001. It deals with major contributions made by the English Courts in the Twentieth Century to three important areas of English contract law. The first is the variation of contracts by subsequent agreement, where developments in the doctrines of consideration and estoppel are discussed. The second is the battle over privacy of contract, and the third is the development of different types of contractual terms.