The incidence of currency counterfeiting and the possible total stock of counterfeits in circulation are popular topics of speculation and discussion in the press and are of substantial practical interest to the U.S. Treasury and the U.S. Secret Service. This paper assembles data from Federal Reserve and U.S. Secret Service sources and presents a range of estimates for the number of counterfeits in circulation. In addition, the paper presents figures on counterfeit passing activity by denomination, location, and method of production. The paper has two main conclusions: first, the stock of counterfeits in the world as a whole is likely on the order of 1 or fewer per 10,000 in both piece and value terms; second, losses to the U.S. public from the most commonly used note, the $20, are relatively small, and are miniscule when only counterfeit notes of reasonable quality are considered.