Over a decade ago the concept of "design experiments" was introduced because of the belief that many of questions could not be adequately addressed by laboratory-based experiments. Since then, design-based research as a term has grown in popularity and significance. The core manuscripts of this special issue respond to the questions: What constitutes design-based research? Why is it important? What are the methods to carry it out? At the end of this issue, two strong commentaries situate this work and challenge the community with new questions and issues that must be answered if design-based research is going to help advance work in ways that others judge as worthwhile and significant.