Since 2000, rents have risen and the number of renters who need low-priced housing has increased. Nationwide, the market provides only 21 adequate, affordable, and available (AAA) units for every 100 renter households with income at or below 30 percent of the area median income (often called extremely low-income, or ELI, renters). Federal assistance adds another 25 AAA units, making a total of 46 adequate and affordable units available to those households. Without the support of federal rental assistance, not one county in the United States has enough affordable housing for all its ELI renters. This report shows national trends in housing affordability for ELI renters, based on data from the 2000 Census and the five-year American Community Surveys for 2005–09 and 2010–14. It updates our 2015 brief, incorporates new data that show the impact of US Department of Agriculture rental assistance programs, and includes a new methodology for integrating survey and administrative data to estimate housing affordability and better isolate the role of federal rental assistance in making housing affordable for ELI households.