What happened when people did not pay their debts? Debts Unpaid argues that conflicts over small-scale unpaid debts were a stress test for the economic order. To ensure the wheels of petty commerce continued to turn in Mexico, everyday debtors and creditors had to believe that their interests would be protected relatively fairly when agreements soured. A resounding faith in economic justice provided the bedrock of stability necessary for the expansion of capitalism over the longue durée. Introducing the two-hundred-year period of massive economic transformation explored throughout the book, this chapter presents the text’s key historical and theoretical interventions from the late eighteenth century to the first decade of the twenty-first. As the capitalist credit economy grew, especially through modern financial institutions, ordinary people used new financial tools and navigated increasingly opaque and impersonal credit relations. This Introduction outlines the dynamics of change and the challenges and opportunities they posed for the world of small-scale debtors and creditors.