Extensive education research on the contribution of teachers to student achievement produces two generally accepted results. First, teacher quality varies substantially as measured by the value added to student achievement or future academic attainment or earnings. Second, variables often used to determine entry into the profession and salaries--including postgraduate schooling, experience, and licensing examination scores--appear to explain little of the variation in teacher quality so measured. (Early experience is the exception.) Together, these findings underscore explicitly that observed teacher characteristics do not represent teacher quality. The precise method of attributing differences in classroom achievement to teachers is the subject of considerable discussion and analysis. The authors begin by briefly outlining the general analytical framework that forms the basis of much of the work in this area and then describe the range of results from recent efforts to measure the variance of teacher effectiveness. (Contains 1 table.) [This paper is published in "The American Economic Review" 100(2), May 2010.]