Trading floors are a thing of the past. Thanks to a combination of computers, high-speed networks and algorithms, millions of financial transactions now happen in fractions of a second. This book studies the automation of stock markets in the United Kingdom and the United States of America, identifying the invisible actors, devices, and politics that were central to the creation of electronic trading. In addition to offering a detailed account of how stock exchanges wrestled with technology, the book also invites readers to rethink the nature of markets in modern societies. Markets, it argues, are sites for the creation of relations, and in studying how these relations changed through technology, the book highlights the sources, dynamics, and consequences of automation. In this respect, the book is both a history of automation in finance and a sociological analysis of the way in which automation gradually changed the lives and work of key financial actors.