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 in the light of Williams v Roffey Bros and Nicholls (Contractors) Ltd and the High Trees case. The second is the battle over privity of contract, discussed mainly in the light of the Midlands Silicone case and Beswick v Beswick; much of this discussion retains its practical importance even after the Contracts (Rights of Third Parties) Act 1999. The third is the development of different types of contractual terms: starting with the distinction between 'conditions' and 'warranties', this discussion traces the development of 'intermediate' or 'innominate' terms in the light of the Hong Kong Fir case and later authorities; it also considers the development and continuing significance of the concept of the 'fundamental' term.