A core component of the No Child Left Behind Act (NCLB, 2001) is the provision of free after-school tutoring services, labeled "supplemental educational services" (SES), to eligible students. Importantly, an explicit NCLB requirement makes each state responsible for evaluating individual SES providers with regard to effectiveness in raising achievement and satisfying clients (parents, teachers, school leaders). In this article, the authors discuss practices and challenges in evaluating SES based on their experiences and findings as SES evaluators in multiple states. Specific topics include: (a) theoretical and empirical considerations in evaluating after-school tutoring, (b) research design and analysis issues, (c) exemplary evaluation practices and findings. Results, in general, indicate mostly positive but very modest measurable effects on tutored students compared to demographically matched "control" students. The implications of such results are examined from the perspectives of (a) establishing meaningful expectations for the contribution that relatively limited hours of tutoring should make to student learning, and (b) improving and broadening evaluation methods to examine implementation and educational outcomes in more highly controlled contexts. (Contains 3 figures, 1 table and 2 footnotes.)