Mathematics and science are important for success in school, but learning mathematics and science can be a difficult task to some students. Efforts to improve students' learning of school mathematics and science have led to reform efforts in curriculum and instruction over the past decades (e.g., National Council of Teachers of Mathematics [NCTM], 2000; National Research Council [NRC], 1996). Yet, in the 2000 National Assessment of Educational Progress, only 17 percent of grade 12 students nationally performed above a basic level of competence. As the improvement of mathematics and science learning is an extremely complex activity that requires coordinated efforts from multiple resources, good research support is very important. In particular, research on students' learning difficulties should help lay a foundation for developing effective interventions, and thus hold much promise for the improvement of teaching and learning school mathematics and science.