The questions, challenges and dilemmas raised at the biennial Evidence and Implementation Summit (EIS) don’t end with the event. Common threads arise – across sectors, professions and places – that continue to resonate long afterward. Over the next few weeks, we'll share some of what has stayed with us: the ideas, tensions and insights from two days of some of the sharpest thinking in our field. First up: scaling. Why is it so challenging, and what do we know about making it work?