When Congress reauthorizes the Elementary and Secondary Education Act of 1965 (ESEA), much of the debate will undoubtedly focus on the accountability requirements added to Title I by the No Child Left Behind Act of 2001 (NCLB). Title I authorizes federal aid to school districts to educate low-achieving children in low-income areas. Among the most controversial of these NCLB requirements are the provisions for determining whether schools and districts have made adequate yearly progress (AYP) in raising student achievement in reading and mathematics. This background paper from the Center on Education Policy (CEP) explores some of the factors that have influenced recent trends in the national percentage of public schools that have not made AYP, out of the total number of schools that reported AYP results. This paper is intended to serve as a companion to the report, "Update with 2009-10 Data and Five-Year Trends: How Many Schools Have Not Made Adequate Yearly Progress?" (CEP, 2011). As discussed in more detail in that five-year trend report, the national percentage of public schools failing to make AYP rose from 29% in 2006 to an estimated 38% in 2010 and actually decreased in two of the interim years. Although 38% is a record high percentage of schools not making AYP, it is still lower than what many observers had predicted by this point in NCLB implementation. CEP's analysis focused on 10 large or medium-sized states that had the greatest increases or decreases in the number of schools not making AYP or had other noteworthy AYP trends: California, Florida, Illinois, Missouri, New York, North Carolina, Oklahoma, South Carolina, Texas, and Washington. Its analysis revealed several factors, in addition to changes in student learning, that appear to account for some of the fluctuations in the national percentage of schools not making AYP and may help explain why these percentages have not escalated as quickly as some analysts have predicted. The main conclusions are as follows: (1) National trends in the percentage of schools not making AYP have been affected disproportionately by trends in a limited subset of states; (2) Changes in state testing policies have slowed, or even reversed, increases in the number of schools failing to make AYP in several states; (3) The NCLB "safe harbor" provision has also helped somewhat in keeping down the share of schools failing to make AYP in certain states; (4) The use of growth models appears to have had a limited impact on AYP trends in most of the growth model states analyzed; (5) In most of the states analyzed, the number of schools failing to make AYP increased substantially in the years when the state's achievement targets went up; (6) States that introduced new tests saw substantial short-term increases or decreases in the number of schools failing to make AYP; (7) In some states, changes in the number of schools not making AYP are largely attributable to changes in the cut scores defining "proficient" performance on state tests; (8) North Carolina's experience suggests that counting proficient scores from retests can reduce the number of schools failing to make AYP--especially in the short term; and (9) Even if most or all states adopt common standards and common assessments, variations in state accountability policies could continue to make it impossible to arrive at meaningful comparisons about the performance of different states. (Contains 6 footnotes.) [For the companion report, "Update with 2009-10 Data and Five-Year Trends: How Many Schools Have Not Made Adequate Yearly Progress?," see ED518993.]