In this chapter, we discuss whether private supplementary education is generating inequalities in terms of educational opportunities and achievement based on a review of previous studies. Specifically, we focus on two perspectives that are highlighted in private supplementary education: access to private supplementary educational opportunities (who is using them), and the effects of private supplementary education (i.e., whether it enhances educational achievement). To examine the impact of private supplementary education on socioeconomic outcomes, it is necessary to conduct a detailed empirical analysis in an effort to verify the trend in private supplementary education use over several decades by income category using microdata that capture the various attributes of households, such as the number of children, and their educational stage, type of school, and grade. We hypothesize in this chapter that the key factor regarding private supplementary education is its manner of use, that is, whether it mainly functions as a means of educational and hierarchical leveling for the lower-middle class, or whether it is linked to the educational and hierarchical reproduction strategy of the upper class.