This essay is based on the Ely lecture, which I delivered at the AEA meeting in January 2017. Many thanks to Alvin Roth,both for his invitation to give this lecture, and for his license to get our hands dirty with policy work. I thank Abhijit Banerjee and Cass Sunstein, master plumbers, for their encouragements, for several conversations that shaped this lecture, and for details comments on a first draft. David Atkin, Robert Gibbons, Parag Pathak, and Richard Thaler, provided useful comments. Vestal McIntyre gave wonderful and detailed comments on style; all awkwardness remains mine. Laura Stilwell provided excellent research assistance. Plumbing is not a solitary activity, and I would like to acknowledge the many people who have plumbed and discussed with me over the years, notably: