This study examines the effect of SES on student test score gains and whether subgroups of students benefit more from NCLB tutoring services, using information about students enrolled in 3rd through 8th grades in 121 elementary and middle schools from 2003-04 to 2007-08. A total of 17 elementary and middle schools were required to offer SES at some point during the period under study, and 9,861 student-year pairings in the sample were eligible to receive SES. The authors find consistently significant and positive average effects of SES on test score gains in mathematics. Results in reading tend to be insignificant. SES tutoring does not appear to disproportionately benefit a particular racial/ethnic group or ability level. Female students and students with disabilities appear to benefit more from participating in SES. SES has a significant, cumulative effect on students in both mathematics and reading. They also demonstrate that not accounting for content area of tutoring can cause downward bias in estimates of the SES treatment effect. These findings are qualified on a couple of dimensions.