This article examines a praxis model of teacher education and advances a new method for engaging novice teachers in reflective practice and robust teacher learning. Social design experiments--cultural historical formations designed to promote transformative learning for adults and children--are organized around expansive notions of learning and mediated praxis and provide new tools and practices for envisioning new pedagogical arrangements, especially for students from nondominant communities. The authors examine one long-standing social design experiment, the UCLA UC Links/Las Redes partnership and the work of one exemplary novice teacher to illustrate the importance of mediated, reflective practices in helping apprentice teachers develop a coherent and orienting framework for teaching and learning that has both heuristic and explanatory power. The authors illustrate how cultural historical concepts of learning and development and situated practice become the means for university students to gain distance and reflect on the beliefs and practices that have informed their understandings of teaching and to "rise to the concrete" practices of learning jointly and resonantly. (Contains 4 notes.)