This article examines the influence of instructional time on achievement, particularly from the perspective of national implementation of schooling and national levels of achievement in mathematics. The report is in four sections. The first section provides a brief introduction to the idea of instructional time as a fundamental educational resource in the implementation of mass compulsory schooling. The second section reviews some past research exploring the relationship between instructional time and achievement. This section ends with a focus on specific research about how the economic development of a country can condition the relationship between instructional time and mathematics achievement across national school systems. The third section presents some original analyses of cross-national data to further illustrate these ideas. The final section provides some policy recommendations. (Contains 14 tables, 2 figures, and 2 notes.)