The Supplemental Educational Services (SES) provision of the No Child Left Behind Act offers free tutoring services to students attending perennially underperforming schools. The author assesses the extent to which Chicago Public Schools students most in need of and who could potentially benefit the most from additional academic instruction participate in SES. The author uses multilevel cohort and longitudinal analyses to explore the characteristics of students participating in SES in Chicago Public Schools from 2004-2005 to 2007-2008. The results of this study, among the first empirical evaluations of SES participation, suggest that students with higher observed cognitive achievement are less likely to engage the SES provision, whereas students with better noncognitive performance are more likely to participate in SES. (Contains 8 tables and 25 notes.)