Academically-focused learning activities beyond formal schooling are expanding in myriad forms throughout the world. This diverse realm of learning activities includes private supplementary education purchased by families such as private tutoring, online courses, cram schools, and learning center franchises. Some public schools also provide academically oriented after-school programs beyond their formal curricula. This review identifies factors relating to students, families, schools, and educational systems that affect participation in supplementary education. Macro forces are also related to the proliferation of learning activities outside of formal schooling. We discuss implications of this trend for educational stratification as well as challenges it creates for families and formal educational systems. Finally, we suggest promising new avenues for data collection and empirical research.