Project STAR (Student/Teacher Achievement Ratio) was a large-scale randomized trial of reduced class sizes in kindergarten through the third grade. Because of the scope of the experiment, it has been used in many policy discussions. For example, the California statewide class-size-reduction policy was justified, in part, by the successes of Project STAR. Recent (failed) proposals in the Senate that sought federal assistance for class-size reductions were motivated by Project STAR research. Even the recent discussion of small schools often conflates the notion of small schools and smaller classrooms. Because of the importance of Project STAR, it has been studied by many scholars looking at a wide variety of outcomes and even exploiting the randomization to understand variations in inputs and other aspects of the education production function that do not directly relate to class size. This paper provides an overview of the academic literature using the Project STAR experiment. Given the scarcity of large-scale educational experiments like Project STAR, it is important to learn as much as possible from the experiment. Researchers have combed through the experiment to learn not only about the effects of reduced class size on test scores but also to gain insight into classroom dynamics. Overall, Project STAR indicates that reducing class size is a reasonable economic investment: the benefits are sizable and long lasting, especially for black students, and the overall benefits outweigh the costs. (Contains 1 figure, 7 tables, and 46 notes.)