The middle ages have long been famous for spectacular forgeries. In this innovative study Alfred Hiatt examines for the first time the function of forged documents in late medieval politics and society. He asks questions such as who would benefit from forged documents, what does a forgery tell us about society at the time and why did forgery so often escape detection? He looks at a wide range of forgeries, including spurious seventh-century papal bulls and the 'Donation of Constantine', arguing that contemporary identification of fake documents was as ideologically motivated as their creation. They emerge as a fascinating paradox - an illegitimate form of historical writing crucial to the expression of personal, institutional and national identities at the time, which are now central to the modern perception of authenticity and value.