The debasement of the Byzantine gold coin during the reign of Constantine IX Monomachus (1042–1055) marked the end of more than seven centuries of near stability and the beginning of a sharp fall in the coin's gold content. I reject a widely accepted view that the debasement was caused by the increase in the number of transactions in an expanding economy. I explain the debasement as being a measure undertaken to finance the protracted war against the Pechenegs in the latter half of Constantine's reign.I would like to thank Judith Herrin for her help and encouragement and for first suggesting that I look at this problem. I would also like to thank this JOURNAL's editor and three anonymous referees whose comments and recommendations significantly improved this article. Finally, thanks are also due to Richard Brealey, Ian Cooper, Michael Crawford, Julian Franks, Jonathan Goodman, Mervyn King, Michael Metcalf, Robert Mundell, Anthony Neuberger, and Gabriel Stein.