Through the post-World War II period, the U.S. dollar has been the leading currency used in international trade and debt contracts. With the creation of the euro, the dollar may finally face a challenge to its dominance in international markets. This article examines the likely effects of the euro on the international use of the dollar. Moreover, the article considers the implications for the United States and the euro area of a rise in the use of the euro and consequent fall in the use of the dollar internationally.