The growth of structured, outside-school activities for improving students' mathematics achievement is an enduring feature of modern schooling with major policy implications. These "shadow education" activities mimic, or shadow, formal schooling processes and requirements. Using extensive cross-national data from the Third International Mathematics and Science Study, we examine shadow education as a macrophenomenon of modern schooling through its (a) prevalence, (b) strategies for use, and (c) associated national characteristics. We find that shadow education is prevalent worldwide, but that there is considerable cross-national variation in its use. Contrary to findings from single country studies, we find most shadow education is remedial in nature. We then test hypotheses concerning the national origins of shadow education and its impact on nations' production of mathematics achievement. Our results show that institutional factors of education, including limited access and lower levels of funding, drive the use of shadow education, instead of high-stakes testing and national achievement incentives. We conclude by discussing implications for both educational policy and theory regarding the degree to which institutionalization of mass schooling increasingly dominates contexts of schooling.