This article exploits highly detailed data on teacher absences from a large urban school district in the northern United States to shed light on the determinants and effects of teacher absences. The topic is important because both school and district policies can influence teachers' propensity to be absent. The authors estimate the impact of teacher absences on academic achievement of students matched to elementary school teachers. Models include fixed effects for teachers to control statistically for potential correlation between time-invariant levels of teachers' skill and effort and their rates of absence. The authors estimate 10 additional days of teacher absence reduce mathematics achievement of fourth-grade students by 3.2% of a standard deviation. They employ an additional instrumental variables strategy to bolster the case for a causal interpretation of results. Instrumental variables results indicate the impact of unexpected teacher absences on student achievement is larger than the impact of anticipated absences. (Contains 7 tables, 1 figure and 20 notes.)