Research on private supplementary tutoring, widely known as shadow education, has a long history but only gathered intensity during the present century. This research has shown much diversity in the scale and nature of shadow education, but further mapping and analysis is needed to reduce gaps in understanding and to keep up with changes. The collection of articles in this special issue of the journal presents insights from parts of Africa, Asia and Europe; and this introductory essay juxtaposes these insights with Hungarian research. The domain of shadow education has many tensions, with both positive and negative implications for individuals, families, the field of education, and societies as a whole. International research helps with understanding these tensions, and in due course with appropriate action to address them. In the process, much can be learned from counterparts in different systems, countries and cultures, not only about the nature and impact of shadow education but also about methodological approaches to research.