HUD's proposal for transforming federal rental assistance expands subsidy recipients' freedom to choose where to live. This essay summarizes research evidence showing that: 1) project-based housing programs limit families' choices about where to live; 2) families benefit when they move with vouchers; 3) assisted mobility programs further expand families' options; and 4) "opportunity moves" can improve families' life chances. Although many families living in federally subsidized housing projects will choose to stay (especially if new investments improve the quality and safety of these communities), expanding opportunities for families to move strengthens federal housing policy by improving the well-being of assisted households.