While policymakers have been directing considerable attention to teacher effectiveness, one basic aspect of effectiveness has received relatively little attention: teacher attendance. No matter how engaging or talented teachers may be, they can only have an impact on student learning if they are in the classroom. This paper asks a simple question: How often are teachers in the classroom and what factors influence their attendance? Using school district data for 40 of the country's largest metropolitan areas for the 2012-2013 school year, the following was found: (1) On average, public school teachers were in the classroom 94 percent of the school year, missing nearly 11 days out of a 186-day school year (the average school year length). Teachers used slightly less than all of the short-term leave offered by the district, an average of 13 days in the 40 districts; (2) 16 percent of all teachers were classified as chronically absent teachers because they missed 18 days or more in the school year, accounting for almost a third of all absences; (3) In spite of previous research to the contrary, this study did not find a relationship between teacher absence and the poverty levels of the children in the school building; and (4) Districts with formal policies in place to discourage teacher absenteeism did not appear to have better attendance rates than those without such policies, suggesting that the most common policies are not particularly effective. The following are appended: (1) Districts included; (2) Methodology; and (3) Findings by district. A Technical Appendix is also included.