A broader understanding of the importance of student behaviors and school climate as drivers of academic performance and the wider acceptance that schools have a role in nurturing the "whole child" have increased attention to indicators that go beyond traditional metrics focused on proficiency in math and reading. The 2015 passage of the Every Student Succeeds Act (ESSA), which requires states to report a nontraditional measure of student progress, has codified this understanding. The vast majority of U.S. states have chosen to comply with ESSA by using measures associated with student absenteeism--and particularly, chronic absenteeism. This report uses data on student absenteeism to answer several questions: How much school are students missing? Which groups of students are most likely to miss school? Have these patterns changed over time? And how much does missing school affect performance? In this report, the authors aim to fill some of the gaps in the analysis of data surrounding absenteeism. They first summarize existing evidence on who misses school and how absenteeism matters for performance. The authors then analyze the National Assessment of Educational Progress (NAEP) data from 2003 (the first assessment with information available for every state) and 2015 (the most recent available microdata). They use this information to describe how much school children are missing, on average; which groups of children miss school most often; and whether there have been any changes in these patterns between 2003 and 2015. The authors also present evidence that higher levels of absenteeism are associated with lower levels of student performance. Major findings include: (1) One in five eighth-graders was chronically absent; (2) absenteeism varied substantially among the groups analyzed; (3) absenteeism varied by state; and (4) prior research linking chronic absenteeism with lowered academic performance was confirmed by the results.