The U.S. needs more affordable housing. A lot more. Between 2001 and 2013 the country lost 2.4 million rental housing units (both market-rate and subsidized) that were affordable to people making less than 50 percent of area median income. Building more units will help, but preserving existing affordable housing is critical too: It’s generally cheaper than new construction, prevents displacement, takes advantage of existing land use patterns and allows people to remain where they already live. But preservation also presents challenges of its own, often necessitating the blending of multiple federal, state and local funding sources and greater collaboration between developers, policymakers and other stakeholders.