One of the great enterprises of the nineteenth century was to systematize the law of contracts. Since the mid-twentieth century, there has been general agreement that the systems have come unstuck. Yet older doctrinal formulations have lived on. Further intricacies have been added to already complicated doctrines. Vague doctrines have replaced rigid ones. The fundamental problem with nineteenth-century contract theory has been sidestepped. Contract was defined in terms of the will of the parties. This theory could not explain why the parties are often bound by terms to which they did not consciously assent, and sometimes they are not bound by harsh terms to which they assented. Contemporary approaches either neglect the idea of fairness entirely or explain it through liberal considerations of choice. Foundations of American Contract Law systematically re-examines the major doctrines of American contract law. It presents an alternative approach that reconciles concerns about fairness, party autonomy, and the purposes that a contract serves for society and the parties themselves. It shows how this alternative better explains the enforceability of contracts, relief for unconscionable terms, the effect of mistake, fraud, duress and changed circumstances, and problems of assent, interpretation, good faith, and remedies for breach of contract.