In this article, the authors review International Large-Scale Assessment (ILSA)-based research over the last several decades, with specific attention on cross-national analysis of mean differences between and variation within countries in mathematics education. They discuss the role of sampling design and "opportunity to learn" (OTL) indicators to assess the use of ILSAs in examining educational inequalities such as tracking and explore improvements in ILSAs' capacity for studying classroom-level differences in instructional content.