The American Economic Association awarded Parag Pathak the 2018 John Bates Clark Medal for his research on the impacts of educational policies. Both the theory and the empirical research take the constraints facing administrators seriously. As a result, Parag’s research led directly to educational reforms in many large US cities and abroad. The leading example is Parag (and co-authors’) research on school assignment mechanisms that led many school districts to institute fairer and more efficient procedures for allocating students to schools. The institutional detail Parag learned in working on the assignment problem led to innovative empirical work on the impacts of different types of schools, most notably of charters, which was suggestive of the characteristics of both successful schools and of the types of students who gained from being enrolled in them. Using the data generated by the new assignment rules, his recent work provides complete frameworks for the quantitative analysis of the benefits of different assignment mechanisms and has measured those benefits in New York high schools.