Regulations requiring the clearing of certain OTC derivatives through central counterparties (CCPs) are causing a profound change in market structure and trading practices. This paper discusses how CCPs are structured and what effects increased use of them will have on the financial system. Craig Pirrong is Professor of Finance, and Energy Markets Director for the Global Energy Management Institute at the Bauer College of Business at the University of Houston. His research focuses on the economics of the organization of financial markets, including the economics of exchange and OTC markets, and the economics of clearing and other mechanisms for allocating counterparty credit risk. He has consulted widely with exchanges around the world, has testified before Congress on energy pricing, and has served as an expert witness in a variety of cases involving derivatives and commodities markets. He holds a Ph.D. in business economics from the University of Chicago.