The overarching goal of the What Works Collaborative is to build knowledge and share solutions with policymakers at the U.S. Department of Housing and Urban Development (HUD) and other federal agencies. During the first phase of the Collaborative, more than two dozen independent research projects were completed on a wide range of current housing and urban development issues. The Collaborative has now turned its attention to fleshing out longer-term policy challenges and identifying research needed to formulate policy to address these challenges. This paper is one of five developed by the Collaborative on different aspects of housing and urban policy. The goal of the paper is to surface critical unanswered questions for policy research and to conceptualize a research agenda to help guide new research initiatives that hold the most promise for moving both policy and practice forward.