This chapter brings together the findings from various sources of academic literature and empirical and statistical evidence in order to answers the question ‘Why do school dropout rates vary (so much) across advanced- and developing countries?’ Answers to this question are sought by using the following sub-questions: (1) What is the definition of school dropout? (2) How big is the problem of school dropout (in advanced and developing countries)? (3) What determines, or ‘triggers’, school dropout? These questions are answered by using post-2010 academic literature on school dropout, published in peer-reviewed journals, and covering both advanced and developing countries. The key findings are summarised as follows. First, close inspection, of definitions stipulated in post-2010 articles, reveals that it is possible to identify explicitly, but also implicitly, three ingredients: (1) a minimum level of education; (2) a relevant age group and (3) those persons who are excluded from calculations. In particular the latter two ingredients can vary (a lot) across studies and official statistics and it does matter for the level of published school dropout rates. Second, although there is large heterogeneity in the level of dropout rates observed across countries, a fairly steady downward sloping trend of the school dropout rate is observed over the past 15 to 25 years for almost all countries over the world. And third, albeit the large set of studies on school dropout determinants, there is still much unknown in the academic literature on the most effective approach for keeping students in school in order to lead them towards a high school diploma. This is mainly because research on the most effective way to tackle early school-leaving was (is) absent.