Building on the claim that teachers need to know how innovative instructional approaches work to be able to adapt them to their own classrooms, design research is presented as a research method that aims to offer exactly that kind of information. We elaborate design research that aims to develop a local instruction theory--a theory about the process by which students learn a given topic in mathematics and theories about the means of support for that learning process. We will illustrate this with the example of design research on a local instruction theory on addition and subtraction to 100. We further discuss design research that combines the 2 goals of teacher learning and student learning in 1 project as a special case of research on teacher learning. In closing, we briefly look into the relation between design research and teacher research. (Contains 7 figures and 1 note.)