.The U.S. government estimates that less than 1/100 of 1 percent (that is, 0.01%), of U.S. paper currency in circulation is counterfeit. Considering recent advances in printing technology and the obviously vast incentive  to counterfeit bills, that is a small number. In part, that's because the U.S. Secret Service thoroughly investigates all reported counterfeiting cases, and because there arecriminal penalties for counterfeiting or passing fake bills. Maybe more than anything, though, counterfeiting is difficult because of the bills' security features, which are hard to reproduce but easy to use to verify your money's authenticity.