Scholars and policymakers see improving teacher quality as a key way to improve student learning. While quality may be improved in a variety of ways for pre-service teachers, professional development is one of the few avenues by which quality can be improved for those teachers already in the teaching profession. But professional development, like other means of improving teacher quality, requires the expenditure of resources. A first step in justifying the expenditures requires understanding the effectiveness of the professional development programs. This article looks at an in-service professional development program targeted towards chronically low-achieving schools in Appalachian Kentucky school districts and examines whether it improved student learning at those schools. We then consider the cost of providing the program and compare the cost effectiveness of professional development to other types of school improvement interventions. Findings suggest that this program is relatively cost effective for improving math outcomes for middle schools. (Contains 7 tables and 29 footnotes.)