He studies market design--an engineering-oriented field of economic theory that considers how the design of the rules and regulations of a market affects the functioning and outcomes of that market. In particular, he studies matching theory, a field of economics that studies markets in which agents have explicit preferences over whom they buy from and sell to, not just over the underlying goods bought and sold. Examples of such economies include the markets for medical residency positions, the assignment of students to public schools, and the formation of collaborative research enterprises. His work in market design has led to a deeper understanding of such diverse markets as kidney exchange, assigning U.S. Army military cadets to branches of service, and the dynamics of shareholder voting.