Designing, facilitating, and scaling-up video-based professional development: supporting complex forms of teaching in science and mathematics.

This special issue brings together four effective video-based professional development programs-two in mathematics and two in science-that use classroom videos as the centerpiece of their efforts and have "scaling up" as their goal. In each paper, the authors surface their design considerations for creating scalable and sustainable video-based professional development interventions, including the challenges and successes they have experienced. The papers also emphasize the role of professional development facilitators in the scalability and sustainability of their programs and how best to support this emerging class of education professionals. This introductory paper situates these papers within the current literature on the design and facilitation of video-based professional development programs.