This thesis examines the historical controversy surrounding the official removal of J. Robert Oppenheimer's security clearance. It argues that the majority finding of the Atomic Energy Commission, which scholarship on the subject has generally rejected, was essentially correct in concluding that Oppenheimer's clearance was removed due to defects in his character. The thesis begins with an analysis of the security hearing itself, revealing how the Personnel Security Board was less concerned with Oppenheimer's loyalty and discretion than with various facets of his character. Much of the testimony dealt with the quality of Oppenheimer's advice, and issues such as his "wisdom," "influence," "arrogance," and lack of "enthusiasm". In order to explain the case against Oppenheimer, this thesis studies the origins and nature of Oppenheimer's character. This begins with an examination of Oppenheimer's childhood and school life, where strong ethical and moral values were instilled in him. It goes on to look at Oppenheimer's entry into the international world of science, and his development of a strong faith in the canons of science. It documents how Oppenheimer, perhaps due to his brilliance, also became something of an intellectual snob during this period, and was often verbally abusive to those less intelligent than him. The burden of this thesis maintains that Oppenheimer represented a way of thinking and of being that was antipathetic to those who dominated the policy-making circles he entered after the war. Oppenheimer's unpopular ideas regarding defense policy, weapons development, and candor in nuclear matters, combined with his arrogance and influence, earned him powerful enemies. Such enemies were responsible for the initiation of his security hearing and the resulting removal of his clearance. This thesis concludes that Oppenheimer's character, which was well suited to the world of science, was completely unsuited to the Cold War policy-making circles he entered after World War II, and that the removal of his security clearance arose out of this discord.