School curriculum and its impact on teaching and learning have received more and more research attention both in the United States and in the international context. For example, the Third International Mathematics and Science Studies (TIMSS) took curriculum and its different forms in the process of curriculum transformation as a guideline to conceptualize the relationship between curriculum analysis and students' learning. As an outline of school educational activities, curriculum also has been the focus of educational reforms in the United States in the past several decades. Because curriculum materials have been a mainstay in mathematics classrooms, it is often assumed that the quality of curriculum materials matters. Thus, efforts to reform curricula in the past often focused on developing or revising curriculum materials for classroom instruction. This article discusses the need for more curriculum research to improve teaching and learning.