This article describes an education design research program that began with individual microgenetic interviews with children in a laboratory setting and led to a developmental model of students' understanding of quotients in mathematics and subsequently to the design and testing of an anchored instruction module for use in whole-class work. The authors discuss the design's theoretical, methodological, and pragmatic aspects. They focus on the development of theory and the generation and refinement of artifacts as mutually constitutive in design research. Special attention is paid to making explicit the causal chain of arguments that link theory development, empirical tests of that theory, and product development. They make the case that effective interventions and rigorous theory can be codeveloped in the design paradigm. (Contains 1 figure and 3 tables.)