In their paper, "Currency Substitution--Unofficial Dollarization and Estimates of Foreign Currency Held Abroad, " Feige, Faulend, Sonje and Sosic estimate that 40% to 60% of existing U.S. dollars circulate outside of the U.S. This estimate is supported by the actions of U.S. federal agencies, which produced posters and pamphlets in 24 languages to highlight the new look and anti-counterfeiting features of updated bills produced between 2003 and 2006.

FORBES: Working For The Yankee Dollar