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Owing to conceptual differences between trade data sources and to statistical problems relating in particular to the Intrastat system for declaring intra-EC trade flows, the level and performance of the euro-zone trade balance are known only with a margin of uncertainty. The first two sections of this article explain in detail the main sources of measurement differences affecting data on international trade, especially within the European Union. This information should be known to any economist wishing to use this type of data in order to avoid serious analytical errors. It is then explained why, as a performance indicator for European monetary union and a factor in trade negotiations, the euro-zone trade balance assumes particular importance, especially in relations between Europe and the United States. At the same time it is pointed out that statistical uncertainties relating to trade data are not the sole prerogative of Europe but are also to be met with in our American partner.