This study explores the effects of High School Equalization Policy(HSEP) in Korea on students’ academic achievement since it began in 1974. Recently, HSEP has received considerable attention and criticisms from policymakers, researchers, and the press. In the study, we first describe the context in which HSEP started and the process of how it has been implemented over the 30 years, then analyze the existing empirical research that investigates the effects on students who attend “equalized” or mixed high schools. We conclude with suggestions on the direction of a future empirical research on HSEP’s effectiveness.