This first article of the special issue presents theoretical and methodological considerations about longitudinally investigating curricular effects on the teaching and learning of mathematics in a cultural context. In particular, in this article, we discuss seven issues related to the investigation of curricular effects on the teaching and learning of mathematics: (1) the need to investigate longitudinally the effects of curriculum, (2) the nature of the reform and non-reform curricula, (3) multiple representations of curriculum use, (4) assessment of classroom instruction, (5) student diversity and achievement gaps, (6) multiple measures of student mathematics achievement, and (7) multi-level analysis of student achievement data. This article not only serves as an introduction to the special issue, but most importantly, also provides a theoretical and methodological context for the two longitudinal projects.