Extended education, on which this volume focuses, is a field with many sub-fields. The Concept Note for the volume embraces multiple forms of learning outside core school hours. While many of these forms are planned by governments and related education authorities, others respond to market forces in largely unplanned ways. This paper focuses on private supplementary tutoring in academic subjects, widely known as shadow education. It commences with a list of drivers of extended education, and recommends elaboration of the list. The paper then turns to specifics of shadow education, analysing the scale, nature and underlying forces in different parts of the world. Similarities across locations partly reflect common features of schooling, markets and social forces, while differences reflect the characteristics of specific societies and the roles of governments. The features have a temporal dimension, with dynamics shaping the scale and nature of shadow education in different ways across time. Looking ahead, private supplementary tutoring will continue to evolve according to changing marketplaces and social dynamics.