Improving students’ learning of mathematics is the major focus of my research. In this chapter, I draw on three lines of research (cross-national studies, longitudinal studies of the effect of curriculum on students’ learning, and longitudinal studies of teachers’ learning to teach through problem posing and their impact on students’ learning) to show the importance of having students’ learning of mathematics at the forefront of research. In particular, I delve deeper into different aspects of the three lines of research that have investigated the impact of different factors on students’ learning of mathematics, from curriculum and instructional quality to teachers’ knowledge and beliefs and even the role of parents in children’s learning. The chapter ends with a discussion of how mathematics education research related to student assessment, classroom instruction, curriculum, and teacher professional learning can center around the mathematics that students learn.