Academic Freedom Protected by Career Security: The Tenure System in the United States Liu Beicheng(98) Academic freedom is the core in the concept of modern university. To ensure academic freedom, under the effort of American Association of University Professors, the tenure system has become a common practice in American universities. With the support of various circles, this system has been functioning properly and has played a significant role in the development of American universities. In recent years, this system has been challenged by marketization, leading to intense debates concerning its pros and cons. This article identifies the essence of the tenure system as a way to protect academic freedom by providing career security, and gives a brief account of the origin and debated issues about the tenure system in American universities.