It is a matter of some difficulty for the English lawyer to predict the effect of a misapprehension upon the formation of a contract. The common law doctrine of mistake is a confused one, with contradictory theoretical underpinnings and seemingly irreconcilable cases. This book explains the common law doctrine through an examination of the historical development of the doctrine in English law. Beginning with an overview of contractual mistakes in Roman law, the book examines how theories of mistake were received at various points into English contract law from Roman and civil law sources. These transplants, made for pragmatic rather than principled reasons, combined in an uneasy manner with the pre-existing English contract law. The book also examines the substantive changes brought about in contractual mistake by the Judicature Act 1873 and the fusion of law and equity. Through its historical examination of mistake in contract law, the book provides not only insights into the nature of innovation and continuity within the common law but also the fate of legal transplants.