Federal Reserve bank notes are widely used outside the United States. Knowing how much U.S. currency is abroad is important for a variety of reasons, but currency movements are notoriously difficult to measure, and estimates of the foreign component of currency stocks and flows have been subject to a great deal of speculation and uncertainty. Here we bring together several new methods and data sources to narrow the range of that uncertainty. According to our estimates, about $200 billion to $250 billion of U.S. currency was abroad at the end of 1995, or more than half the roughly $375 billion then in circulation outside of banks. Moreover, that proportion has been rising. Our calculations indicate that growth in foreign demand for U.S. currency–especially for hundred-dollar bills ($100s)–is far stronger than growth in U.S. demand. On average over the 1990s, the overseas stock has been growing at about three times the rate of growth of the domestic stock.