The United States is one of the last countries to tax married couples jointly; most other countries have adopted individual taxation. In 1990, the United Kingdom completed transitioning its tax system from one that treated husbands and wives as a marital unit to one that mandates an individual-based system, and so it has two decades of experience with the new regime. This article provides American policymakers valuable information regarding the consequences of adopting individual taxation by examining the U.K.’s experience. First, it establishes a matrix of factors that identifies and assesses differences between the two nations that affect the predictive value of the U.K.’s experience for the U.S. The article then reviews the origins and development of the U.K.’s original mandatory joint return and the forces that drove the change to individual taxation. Finally, it appraises the consequences of this revision of British law, including the improved economic position of many wives and the increased incidence of tax avoidance. Comparing the change in the U.K. with what would likely occur in the U.S., this article uses comparative taxation to provide a guide for the U.S., urging consideration of the costs as well as the benefits of changing tax units.