The debate about how to measure and quantify the impact of professional learning on teachers and students is often peppered with statements about how we don’t know what works. In fact, many studies that examine a specific strategy or program, often as a component of a larger professional learning system, find measurable positive effects. These studies are important to educators, professional learning advocates, and the field as a whole. Understanding both the specificity and the generalizability of such studies helps build an awareness that professional learning is critical and has impact, especially when we carefully consider which strategies work for which educators and students under what conditions. Learning Forward is working to bring precision to conversations about the outcomes of professional learning, especially by examining