This paper views teacher quality through the human capital perspective. Teacher quality exhibits substantial growth over teachers’ careers, but why it improves is not well understood. I use a human capital production function nesting On-the-Job-Training (OJT) and Learning-by-Doing (LBD) and experimental variation from Glewwe et al. (2010), a teacher incentive pay experiment in Kenya, to discern the presence and relative importance of these forces. The identified set for the OJT and LBD components has a closed-form solution, which depends on experimentally estimated average treatment effects. The results provide evidence of an LBD component, as well as an informative upper bound on the OJT component.