School districts required under No Child Left Behind to provide supplemental educational services (SES) to students in schools that are not making adequate yearly progress rely heavily on the private sector to offer choice in service provision. If the market does not work to drive out ineffective providers, students will be less likely to gain through their participation in SES. We estimate SES provider effects in an urban school district that accounts for a significant share of participating students. Using four different estimation techniques, we identify variation in the effectiveness of different types of providers, as well as across specific providers, in increasing students' math and reading achievement. We expect this research to inform education policy, including the reauthorization of Title I and district tutoring interventions both under NCLB and following federal waivers from NCLB, and to generally address the viability of policy interventions that employ a private market model to improve public sector outcomes. [This research was funded by the Institute for Educational Sciences, PR/Award number: R305A090301, Education Policy, Finance and Systems Research Program, Goal 3.]