This article addresses the teaching and learning side of the learning progressions literature, calling out for measurement specialists the knowledge most needed when collaborating with subject-matter experts in the development of learning progressions. Learning progressions are one of the strongest instantiations of principles from "Knowing What Students Know", requiring that assessments be based on an underlying model of learning. To support student learning, "quantitative" continua must also be represented "substantively", describing in words and with examples what it looks like to improve in an area of learning. For formative purposes, in fact, "qualitative" insights are more important than "scores". By definition, learning progressions require iterative cycles of development so as to build in horizontal coherence among curriculum, instruction, and assessment. Learning progressions are also an important resource for teacher learning. With accompanying professional development and institutional supports, they can help teachers engage their students in richer and more equitable learning experiences. Examples are cited whereby learning progressions can be used to help teachers improve their skills in setting learning goals, interpreting student ideas in relation to a progression, and responding to student ideas with specific interventions that serve to move learning forward.