Addressing student absenteeism continues to permeate education policy and practice. California and a majority of other states have incorporated "chronic absenteeism" as an accountability metric under the Every Student Succeeds Act. It is therefore a crucial time to take stock of what we know on the research, policy, and practice to better understand the measurement of student absenteeism and how to reduce it. To further this goal and spark a broader conversation about student attendance as a valuable policy lever, we wrote the first book centered on the issue of school absenteeism. This policy memo summarizes our multifaceted, multidisciplinary examination of what we have learned about how schools measure and reduce absenteeism and what we need to know going forward as California and other states hold schools and districts accountable for students' absences.