The international comparative studies in 1959 were conducted by International Association for the Evaluation of Educational Achievement (IEA) researchers who recognized that differences in student achievement measures in mathematics across countries could be caused by differences in curricula. The measurements of opportunity to learn (OTL) grew from a small effort in 1959 to a much larger efforts by 1995 to explain whether countries with high achievement were more likely to teach advanced mathematics. In general, the relationship of coverage of a mathematics topic was weakly related to the level of mathematics performance, but did have some effect on growth. This paper finds that differences in measurement methods of OTL across the studies greatly affected the outcome of the relationship. Recent Program for International Student Achievement (PISA) analyses indicate that the relationship between OTL and student achievement might be described as a curvilinear relationship. Countries with lower achievement are more likely to be affected by curriculum coverage than are high-performing countries.