Using evidence of student understanding is crucial for teachers to improve instruction, yet accomplishing this in the current U.S. assessment context comes with numerous challenges, such as resource and time constraints as well as variation in teacher knowledge (Bennett, 2011; Riggan & Oláh, 2011). Bennett (2010) proposes leveraging a domain-specific competency model and associated learning progressions as one way to inform assessment design, curriculum, and teacher professional development so that these components can better impact classroom practice. The purpose of this paper is to present Cognitively Based Assessment of, for, and as Learning (CBAL) as a model for linking a learning progression to classroom assessment in mathematics. A series of computer-based formative tasks designed to assess 4th- and 5th-grade students’ understanding of rational number were developed from a learning progression (Arieli-Attali & Cayton-Hodges, in press) and were piloted in classrooms in 2014. Results from student work as well as from educator interviews will speak to the utility of the learning progression and the tasks to inform teacher interpretation of student understanding and instructional modification. We will also discuss the challenges that accompany such work as well as modifications to materials that may help strengthen the link between assessment and instruction.