Many important economic variables are subject to revision. This article explains how, when, and why such revisions occur; how revisions to Canadian gross domestic product (GDP) compare with GDP revisions in some other countries; which GDP components are subject to the largest revisions; and how data revisions can affect policy decisions. The author finds that revisions to Canadian GDP tend to be smaller, on average, than those of some other countries, and that among the GDP components, exports and imports are most heavily revised.