This dissertation examines the practice and prosecution of bigamy in fifteenth-century Northeastern France. A study of court actions involving concurrent marriage; what we would call bigamy, is essential to our understanding of late-medieval law and society, offering a new account of “the crisis of marriage” in the century leading up to the Reformation. The dissertation is founded on primary research in court records of the diocese of Troyes. It begins, however, with theology and culture. Christian doctrine defined marriage as both monogamous and indissoluble, for at least as long as both spouses lived. This concept of marriage abhorred any division or duplication. A marriage could not end by divorce, nor could it be duplicated by another, concurrent marriage. That does not mean that many men and women did not try to marry despite being already married to a living spouse. How and why they did so, I argue, is of great importance to our understanding of marriage practices among ordinary Christians in the fifteenth century. How and why the ban on bigamy came to be enforced in court, with harsh punishment, is more important still. The fifteenth-century records of the bishop's judicial court in Troyes include over seventy such cases. Twenty of the men and women suspected of bigamy received strikingly harsh punishment in the context of usually more lenient ecclesiastical justice: public humiliation on the ladder on a scaffold, and imprisonment for some months. By examining these cases, this dissertation reconstructs the aims and values of Northern French society on the eve of the Reformation. This dissertation is a contribution not only to social history, but also to the analysis of marriage law. Prior scholarship has laid much emphasis on the problem of “clandestine marriage,” marriages contracted informally, and on marriage formation in general. This dissertation introduces a considerable change in emphasis, arguing that problems such as clandestine marriage, as reflected in the records of Troyes, turned far more on cases of bigamy than has been understood. Multiple marriage was both inevitable and repugnant in a Christian world that forbade divorce and associated bigamy with unchristian practices.