Why should the law enforce contracts? This book argues that contract law exists primarily to support markets but that markets should be understood as normatively desireable for reasons beyond mere economic efficiency. This book thus updates the doux commerce argument of 18th-century theorists such as Montesquieu and Adam Smith, who saw markets as an important and largely positive moral and political influence. Commerce tends to inculcate a set of moral habits -- virtues -- that support liberal political regimes by forcing market participants to view the world from another's point of view. Markets provide a framework for social cooperation that does not require deep moral or political agreement or tribal solidarity. Finally, well-functioning markets produce wealth and prosperity, which can have an ameliorative effect a host of social evils. Contract law tends to strengthen and extend markets and is valuable for that reasons. Placing markets at the center of the normative defense of contract law has concrete implications for how we should think about doctrinal issues such as contract formation, remedies, and the treatment of boilerplate agreements. In all of these contexts, it shifts attention away from the highly individualistic focus of much of contemporary contract law theory and toward system concerns with how contract law structures markets. Finally, because markets at times become pernicious, this book's argument also provides insight into the limits of contract law.