Collaborative design in which both teachers and researchers participate can be a powerful context for capacity- and knowledge-building. However, typically the focus is on what and how teachers learn in co-design efforts. As important is how researchers learn through co-design with teachers. Co-design is an expansive learning context because it juxtaposes goals, purposes, and outcomes of multiple participants and communities. Emergent tensions expand opportunities for learning of all participants and communities. The chapters in this volume exemplify a range of co-design contexts and thereby illuminate different expansive learning contexts for teachers and researchers. This book was originally published as a special issue of "Cognition and Instruction."