What Were Learning from Researchers of Teaching In the past, the question about the mathematical preparation of teachers has turned on quantity: How much mathematics do teachers need to know? This led people to look at the mumber of mathematics courses taken. The assumption was that teachers simply need more of the standard offerings classes typically designed with mathematics, engineering, business, and other students in mind. By looking at the mathematics that teachers need, however, we open the possibility that the mathematical content of courses for teachers might be different from existing offerings. By studying mathematical dimensions of the work teachers do, researchers have provided evidence that it is not simply a question of how much mathematics teachers need to know. Instead, the issue is one of understanding what mathematics plays a role in teaching and when and where it plays that role.