It is one of the happy incidents of the federal system that a single courageous State may, if its citizens choose, serve as a laboratory, and try novel social and economic experiments without risk to the rest of the country. --Supreme Court Justice Louis Brandeis, dissenting opinion in New State Ice Co. v. Liebmann 285 U.S. 262 (1932) In 2006, Massachusetts enacted a comprehensive package of health insurance reforms intended to achieve universal coverage. The basic framework of these reforms--Medicaid expansions, a health insurance exchange, subsidies for lower-income households buying private coverage, an individual mandate, and an assessment on employers who do not provide insurance coverage--became the model for the coverage expansions in the Affordable Care Act (ACA) enacted in March 2010 and scheduled to take effect in each state in 2014. As states proceed with the implementation of these reforms, they may look to Massachusetts for some indication of what to expect as the ACA coverage expansions take effect. One question that readers will likely have is how relevant the Massachusetts experience is for other states. This question has two components. The first is whether the Massachusetts reform was really the same as the ACA. The answer for the most part is yes, although there are some differences. Table 1 outlines key provisions of both reforms, which are the same, and notes where there are differences in, for example, income thresholds for subsidy eligibility or the size at which employers not offering insurance face penalties. In most cases, these differences are relatively minor--for example, the fact that there are four tiers of generosity for exchange plans in the ACA but only three in Massachusetts--and should have little effect on generalizing the findings from Massachusetts to other states. The second factor that affects the generalizability of the Massachusetts experience is whether Massachusetts is a typical state in other ways. Here, the answer is a bit different, because Massachusetts is clearly not a typical state. Its population is relatively well-educated and well-off, with the highest fraction of adults with a bachelor's degree or more and the sixth-highest median household income (Statistical Abstract 2012, Tables 233 and 706). Prior to reform, the uninsurance rate in Massachusetts was already low--10.7% compared with 15.7% nationally--although six states (Hawaii, Iowa, Maine, Minnesota, New Hampshire, and Wisconsin) had lower rates (DeNavas-Walt, Proctor, and Lee 2006, Table 10). In addition, Massachusetts already had enacted significant insurance market regulations, such as guaranteed issue and community rating, so that establishing an exchange and imposing a mandate were the only missing pieces of the puzzle in the individual insurance market. One might argue that Massachusetts, with relatively more resources and fewer uninsured people than other states, was uniquely situated to address the problem. Even if Massachusetts is not a typical state, its experience with reform provides useful lessons for other states. With the Affordable Care Act's infusion of federal assistance through premium tax credits and an enhanced Medicaid match, even states that have few resources of their own to devote to the uninsured are likely to realize substantial reductions in their uninsured populations, just as Massachusetts did, with attendant improvements in access and health. And for those states that are interested primarily in learning from the design and functioning of the exchange in Massachusetts, there is no reason to think that the fundamentals of consumer demand and insurer supply in the individual market are different there than they would be in another state's exchange. Lessons from Massachusetts about exchange design are therefore particularly valuable. The goal of this special section in Inquiry is to provide a concise and accessible summary of some of the major lessons to date from the Massachusetts reform. …