The so-called shadow education system of private supplementary tutoring has become a major phenomenon in all world regions. The metaphor of the shadow is used because much of the tutoring curriculum mimics that in regular schooling: as the curriculum changes in the schools, so it changes in the shadow. This chapter outlines the scale of shadow education in a range of countries, and considers the core factors shaping demand and supply. Subsequent sections focus on costs and effectiveness. Although shadow education consumes large portions of household budgets as well as much time from tutors, students and parents, research from an economic perspective is underdeveloped.