This publication brings back into print the complete testimony, as released by the AEC in 1954, of the hearing called to determine if Oppenheimer was a ""security risk."" But the importance of the document goes far beyond the considered question of whether to terminate his security clearance: The testimony spans Oppenheimer's personal life from the 1920s to the 1950s, reflects the broader political and scientific stirrings of this period, and raises issues that remain central today. The document develops all these themes-it is a compelling human portrait, an eyewitness history of some of the most important events of the century, and a flashback to one of the points of origin of our present concerns with the arms race and government decision making, conscientious dissent and national loyalty. The testimony is inherently dramatic (as a recent play of the same title has shown). In the title role, Oppenheimer reveals himself as a man of Hamletlike complexity, by turns humble and arrogant, naive and knowing, candid and reserved, witty and deadly serious. His final greatness makes itself felt in that joining of resignation to resolution with which he accepts the adverse outcome of his case (to him, inevitable) and its tragic implications for the nation. As background, the testimony carries the reader through a number of settings: the revolution in physics of the late 1920s; the depression, the rise of Nazism, the Spanish Civil War, and the American left's fascination with another sort of revolution; the war years, the building of the atomic bomb, and the decision to use it; the simultaneous opening of the U.N. and the Cold War, and the failure to bring atomic weapons under international control; and the heyday of McCarthyism with its patriotic hysteria, suspicion, and repression. The abiding pertinence of this document is obvious. It is addressed to a nation loudly debating ABM and MIRV deployment, leftist politics, military-industrial-governmental power, the question of loyalty, the limits of dissent, and the right of a man to define for himself, in conscience, what the ""national interest"" is or should be, as Oppenheimer did in opposing the creation of the hydrogen bomb. Among those called to give witness in these matters are Fermi, von Neumann, Bethe, DuBridge, Rabi, Teller, Gen, Groves, McCloy, Lilienthal, Karl Compton, Bush, and Conant. For the first time, the transcript has been provided with an index.