Ali Hortaçsu is the Ralph and Mary Otis Isham Professor of Economics at the University of Chicago. Much of his work is focused on empirically assessing the efficiency of markets. In a nutshell, he utilizes detailed micro-level data from the markets he studies to estimate preference and technology parameters that rationalize individual behavior. He then uses the estimated preferences and technological parameters to construct (constrained) efficient benchmarks and assess how far observed market outcomes are from efficiency. This comparison also motivates discussions of how market rules can be altered to improve efficiency. He has applied the above framework to many market settings, including financial markets, energy markets, and the Internet, and a variety of market clearing mechanisms, including auctions, matching, and costly search.