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 Colorado'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 Colorado's system is compared with other states. Colorado is a state with a set of rules, which, when working in tandem, put the state in the middle of the sample distribution in terms of how many schools make AYP. Although Colorado's proficiency standards are relatively easy to achieve (all of them are at or below the 25th percentile in both reading and math), its annual targets for proficiency are fairly ambitious. Colorado's minimum subgroup size is 30, smaller than most other states examined. Because Colorado has more subgroups to account for than schools in most other states, schools large enough to have many accountable subgroups fail to make AYP while small, homogenous schools tend to make AYP, even if overall student achievement is lower. Compared to the other 27 states examined, Colorado places in the upper middle of the distribution in terms of the number of schools making AYP. The report concludes that although in some respects, Colorado'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 12 footnotes, 6 figures, and 6 tables.) [For "The Accountability Illusion," see ED504712. For individual state reports, see ED504680-ED504694, ED504700-ED504711 and ED504719.]