Once a market in which human beings traded at human speeds, the U.S. stock market is now an electronic market pervaded by algorithmic trading, conducted at nearly the speed of light. High-frequency traders participate in many transactions, and a significant number occur on alternative trading systems known as “dark pools.” Merritt Fox, the Michael E. Patterson Professor of Law at the Law School; Lawrence Glosten, the S. Sloan Colt Professor of Banking and International Finance at the Business School, and Gabriel Rauterberg, an assistant professor at University of Michigan Law School, offer a comprehensive look at how these markets work, how they fail, and how they should be regulated.