The 2001 renewal of the United States' Title I program instituted the Supplemental Educational Services (SES) program, through which students attending schools that fail to make adequate yearly progress (AYP) for 3 consecutive years are offered tutoring in core subjects. Although some SES evaluations have measured provider effects on student achievement, there has been no comprehensive synthesis of SES effects and there is no systematic information relating provider characteristics to variations in student achievement effects. This study synthesized provider effects reported in the extant body of SES provider evaluations to generate an estimate of the overall effectiveness of the SES policy and to identify provider characteristics that are associated with variation in student achievement effects. (Contains 8 tables and 5 figures.)