The extensive investigation of the contribution of teachers to student achievement produces two generally accepted results. First, there is substantial variation in teacher quality as measured by the value added to achievement or future academic attainment or earnings. Second, variables often used to determine entry into the profession and salaries including post-graduate schooling, experience, and licensing examination scores appear to explain little of the variation in teacher quality so measured with the exception of early experience. Together these findings underscore explicitly that observed teacher characteristics do not represent teacher quality. Education production function research on the measurement of teacher value added to student achievement represents a shift from a research design that focuses on the link between student outcomes and specific teacher characteristics to a research framework that uses a less parametric approach to identify overall teacher contributions to learning. Using administrative data bases, some covering all of the teachers in a state, such research provides strong support for the existence of substantial differences in teacher effectiveness, even within schools. Although this approach circumvents the need to identify specific teacher characteristics related to quality, the less parametric approach introduces additional complications and has sparked an active debate on the measurement and subsequent policy use of estimated teacher value added.