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 Idaho'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 Idaho's system is compared with other states. Only two elementary schools and none of the middle schools in the selected sample made AYP in 2008 under Idaho's accountability system. Idaho's relatively small minimum subgroup size of 34 means that schools in Idaho will be accountable to more subgroups than would similar schools in other states with higher subgroup sizes. Many disadvantaged subgroups failed their annual targets in Idaho, as well as quite a few white subgroups in reading. Another factor which makes it difficult for schools to make AYP in Idaho is that no confidence interval is applied to proficiency rate calculations. Compared to the other 27 states examined, Idaho places in the lower end of the distribution in terms of the number of schools making AYP. The report concludes that although in some respects, Idaho'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.]