Sidney Hook set the terms of debate on Communism, higher education, and academic freedom in the postwar United States. His view that Communists lacked the independence necessary for teaching and research--a view forged in the heated debates of New York City's radical left in the 1930s--provided the rationale for firing Communist professors across the country in the late 1940s and 1950s. Relying on close readings of underutilized archival sources, this article explores the development of Hook's thinking, charts his impact on key players in the period's higher education establishment (such as philosopher John Dewey and the American Association of University Professors), and outlines the way his writings helped lead to faculty dismissals at the University of Washington and New York University. The article also highlights the work of students and professors who challenged Hook's anti-Communist position, revealing a rich and often neglected mid-century discourse on academic freedom.