Learning progressions detail the incremental steps that students take as they learn to master a skill. These progressions are based on developmental research about how students learn and how their thinking develops as a result of instruction. A typical progression not only describes the stages that students must master, but it also shows what students can perform at any given level. In this article, the authors describe a study in which teachers study a mathematics learning progression for measuring area and use that progression to analyze student work. Instead of simply noticing the strategies a student used to complete a work sample, the learning progression allows teachers to place the students' understanding at specific stages of development. With that information, teachers can make targeted instructional decisions to advance students to the next stage--and ultimately to the mastery of the content.