The central question in securities-market structure is whether investors are better served by human-mediated markets, such as the specialist system of the New York Stock Exchange, or by electronic markets, such as NASDAQ and the Electronic Communication Networks. The Securities and Exchange Commission's recent Regulation NMS, although it purports to be a reform measure, fails to address this basic issue. However, close analysis of how the different markets function under varying circumstances can shed light on the optimal structure for the securities markets in the United States. At this conference, as part of AEI's ongoing review of securities market structure, we will consider an important study of these markets by Kenneth M. Lehn and his two colleagues at the University of Pittsburgh.