The Affordable Care Act (ACA) will make the labor supply, measured as the total compensation paid to workers, 0.86 percent smaller in 2025 than it would have been in the absence of that law, the Congressional Budget Office estimates. Three-quarters of that decline will occur because of health insurance expansions, which raise effective tax rates on earnings from labor—for instance, by phasing out health insurance subsidies as people’s income rises—and thus reduce the amount of labor that workers choose to supply. The labor force is projected to be about 2 million full-time-equivalent workers smaller in 2025 under the ACA than it would have been otherwise. Those estimates were based mainly on CBO’s calculations of the effects of the law’s major components on marginal and average tax rates and on the agency’s analysis of research about the change in the labor supply resulting from a change in tax rates. For components of the law that were difficult to express in terms of changes in tax rates, CBO based its estimates on a review of the available literature about similar policy changes.