A learning trajectory describes the progression of student thinking and strategies over time in terms of sophistication of both conceptual understanding and procedural fluency. Currently, learning trajectories exist in the research literature for many mathematical domains, including counting, addition and subtraction, multiplicative thinking, measurement, geometry, proportional reasoning, and algebraic thinking. These trajectories are at the heart of the Common Core Standards and elucidated in the companion Progressions documents (Common Core Standards Writing Team 2013). This article describes tools and routines developed by the Ongoing Assessment Project (OGAP) that make these trajectories accessible for teachers and can enhance both instructional practice and student learning. In particular, the authors illustrate how the OGAP Proportionality Progression can help teachers draw on this research to make sense of and respond to student thinking about proportional reasoning in the middle grades. [This article was edited by Natasha Murray.]