The Affordable Care Act (ACA) was enacted in March 2010, and most of its features went into effect in fall 2014. It is the most dramatic change in US healthcare policy since the enactment of Medicare and Medicaid in 1965. describe the key features of the law, focusing on the individual and employer mandates, the insurance exchanges, and the Medicaid expansion. As part of this discussion, we will describe how the ACA implementation unfolded, how it was intended to be financed, and the early empirical literature on the effects of the law. This chapter takes an overview of these effects; in most of the succeeding chapters, there will be opportunities to examine how the law changed previous practices and what effects the law has had or is expected to have on the insurance market. We will also introduce many of the key changes in the administration of the law that have been introduced by the Obama and Trump administrations and by Congress.