Abstract This paper reviews research on private supplementary tutoring, widely known as shadow education, during the initial decades of the present century. It takes as its starting point the first global study of the phenomenon, published in 1999, though notes some scattered national and subnational literature prior to that date. During the initial two decades, great expansion of the research on shadow education brought more depth and stronger awareness of commonalities and differences in different cultures. From initial mapping and identification of factors shaping demand came work on ecosystems with deeper sociological and economic analyses, together with greater attention to research methods. The agenda ahead will need to keep up with changing times, e.g., through the impact of technology, and develop stronger interdisciplinarity to explore additional domains. It will also need continued attention to definitions and methods.
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