The intent of the No Child Left Behind (NCLB) Act of 2001 is to hold schools accountable for ensuring that all their students achieve mastery in reading and math, with a particular focus on groups that have traditionally been left behind. Under NCLB, states submit accountability plans to the U.S. Department of Education detailing the rules and policies to be used in tracking the adequate yearly progress (AYP) of schools toward these goals. This report examines Illinois's NCLB accountability system, particularly how its various rules, criteria and practices result in schools either making AYP, or not making AYP. It also gauges how tough Illinois's system is compared with other states. Illinois has relatively low proficiency standards: in reading, cut scores range from the 35th to the 22nd percentile, and in math, cut scores range from the 20th to the 15th percentile. Illinois schools also have fewer accountable subgroups than in most other states because Illinois's minimum subgroup size (45) is a bit higher than the minimum in many other states. With fewer accountable subgroups and relatively easy proficiency standards, it is possible for more study schools to make AYP in Illinois. Compared to the other 27 states examined, Illinois places towards the upper end of the distribution in terms of the number of schools making AYP. The report concludes that although in some respects, Illinois's NCLB accountability system is working as intended by the legislation, NCLB shortcomings are also evident, and should be considerations for Congress in future reauthorizations of the act. (Contains 9 footnotes, 4 figures, and 6 tables.) [For "The Accountability Illusion," see ED504712. For individual state reports, see ED504680-ED504694, ED504700-ED504711 and ED504719.]