As the 2020–21 school year began, students oscillated among virtual, hybrid, and in-person learning as districts and states balanced the need to keep students and staff safe with the need to provide an effective learning environment. Students faced multiple schedule changes, were assigned new teachers midyear, and struggled with glitchy internet connections and Zoom fatigue. This was a uniquely challenging year for teachers and students, and it is no surprise that it has left its mark—on student learning, and on student well-being. As we analyze the cost of the pandemic, we use the term "unfinished learning" to capture the reality that students were not given the opportunity this year to complete all the learning they would have completed in a typical year. Some students who have disengaged from school altogether may have slipped backward, losing knowledge or skills they once had. The majority simply learned less than they would have in a typical year, but this is nonetheless important. Students who move on to the next grade unprepared are missing key building blocks of knowledge that are necessary for success, while students who repeat a year are much less likely to complete high school and move on to college. And it's not just academic knowledge these students may miss out on. They are at risk of finishing school without the skills, behaviors, and mindsets to succeed in college or in the workforce. An accurate assessment of the depth and extent of unfinished learning will best enable districts and states to support students in catching up on the learning they missed and moving past the pandemic and into a successful future.