The FBI files on Muhammad Ali in the two volumes which make up this collection were released in four batches from December 2016, six months after the boxer's death. While some FBI documents on Ali are still unavailable, there remains a massive body of material of almost 1,000 pages, reproduced here in its entirety. In his extensive Introduction, Josh Keen seeks to explain the FBI's motivations for recording the activities of the late boxer throughout the 1960s and beyond, and outlines the main factors which caused him to be considered politically subversive and worthy of such surveillance. Keen is the first to analyse them in a scholarly way in their biographical and historical context, and shows that no one has yet really come to grips with the Ali evidence and how much it contributes, not only to Ali's biography but also to our understanding of the FBI in the period and within the wider history of the 1960s. He argues convincingly that the FBI's attention to Ali was not merely part of its broader investigation of the Nation of Islam movement to which Ali belonged. It was pursued and extended because of Ali's outspoken challenges to white America and the Vietnam War and the effect his words and actions had as the most famous sportsman in the world. By engaging with the historiography on the FBI, Keen explores how perceptions of the federal agency have changed as its shield of secrecy has been gradually dissolved. Particular focus is placed upon the overbearing influence on its activities of the Bureau's Director, J. Edgar Hoover, especially in its counter-intelligence program. Keen shows how Ali acted 'subversively' on multiple fronts, and how these must therefore be seen in the context of Hoover's own personal beliefs about what America should look like and what sort of behaviour could be tolerated. Exploring the FBI's reaction to Ali's involvement in the 'semi-religious group', the Nation of Islam, reveals Hoover's anxieties over Ali's devotion to Elijah Muhammad's interpretation of Islam, as well as his radical standpoint on race relations. Keen's analysis of the correlation between FBI surveillance and Ali's increasingly public anti-Vietnam War stance also highlights the significance of Ali's politically symbolic refusal to join the armed forces, and of the movement that he stirred.