High-frequency trading (HFT) is a broad term without a precise legal or regulatory definition. It is used to describe what many characterise as a subset of algorithmic trading that involves very rapid placement of orders, in the realm of tiny fractions of a second. This book provides an overview of HFT in the equities and derivatives markets regulated by the SEC and the CFTC. It also examines the Flash Crash of 2010 and the role that HFT may have played, as well as recent regulatory developments.