Despite earlier forecasts to the contrary, US health care spending growth has slowed in the past four years, continuing a trend that began in the early 2000s. In this article we attempt to identify why US health care spending growth has slowed, and we explore the spending implications if the trend continues for the next decade. We find that the 2007–09 recession, a one-time event, accounted for 37 percent of the slowdown between 2003 and 2012. A decline in private insurance coverage and cuts to some Medicare payment rates accounted for another 8 percent of the slowdown, leaving 55 percent of the spending slowdown unexplained. We conclude that a host of fundamental changes—including less rapid development of imaging technology and new pharmaceuticals, increased patient cost sharing, and greater provider efficiency—were responsible for the majority of the slowdown in spending growth. If these trends continue during 2013–22, public-sector health care spending will be as much as $770 billion less than predicted. Such lower levels of spending would have an enormous impact on the US economy and on government and household finances.