TheContaduría Mayor de Cuentas of Castile—that is, the Audit Board of the Accounts of Castile—is one of the earliest supreme state auditing institutions of which we have a documentary record. This article discusses what we know about the organization and functions of theContaduría Mayor de Cuentas of Castile on the threshold of the modern age, in the hopes of finding some elements that can be incorporated into, and so enrich, our theoretical approach to the economic functions of supreme auditing institutions.