Organizations, programs, projects, investors around the world aim to achieve 'impact' with their efforts. The concept of ‘scale’ is an important one in this effort to program designers and evaluators alike, in many fields of practice. This panel will expand the discussion on how development actors might invest in innovation and research for large-scale positive change. As central themes the panel will seek to refine and develop the understanding of how ‘impact at scale’ might be defined, designed, monitored and assessed. Concepts of further discussion may include hotly contested issues such as: pathways to scale, optimality, evidence, unintended consequences. Commentary from international leaders will supplement discussion from 4 organizations working in the innovation for positive social impact space: IDRC, USAID, DFID and the Rockefeller Foundation.