This thesis analyzes the origins, growth, and legitimization of the standardized test preparation ("test-prep") industry from the late 1940s to the end of the 1980s. In particular, this thesis focuses on the development of Stanley H. Kaplan Education Centers, Ltd. ("Kaplan") and The Princeton Review ("TPR"), and how these companies were most conducive in making the test-prep industry and standardized test-preparation itself socially acceptable. The standardized test most frequently discussed in this thesis is the Scholastic Aptitude Test ("SAT"), especially after its development came under the control of Educational Testing Service ("ETS"), but due attention is also given to the American College Testing Program ("ACT"). This thesis argues that certain test-prep companies gained legitimacy by successfully manipulating the interstices of American business and education, and brokered legitimacy through the rhetorical devices in their advertising. However, the legitimacy for the industry at-large was gained by default as neither the American government nor the American public could conclusively demonstrate that the industry conducted wholesale fraud. The thesis also argues that standardized test manufacturers were forced to engage in a cat-and-mouse game of pseudo-antagonism and adaptation with the test-prep industry once truth-in-testing laws prescribed transparent operations in standardized testing. These developments affect the current state of American standardized testing, its fluctuating but ubiquitous presence in the college admissions process, and the perpetuation of the test-prep industry decades after its origins.