Since 2002, the No Child Left Behind Act (NCLB) has spurred far-reaching changes in elementary and secondary education, all aimed at accomplishing the same fundamental goal--to improve students' academic achievement. As the Congress prepares to reauthorize the Act, two related questions matter most: (1) Has student achievement in reading and math increased since NCLB was enacted?; and (2) Have achievement gaps between different subgroups of students narrowed since NCLB was enacted? To answer these questions, the Center on Education Policy (CEP), an independent nonprofit organization, conducted the most comprehensive study of trends in state test scores since NCLB took effect. The authors carried out this study with advice from a panel of five nationally known experts in educational testing or policy research, and with extensive technical support from the Human Resources Research Organization (HumRRO). Although they collected data from all 50 states, not every state had enough consistent data to do a complete analysis of test score trends in reading and math before and after 2002. Based on the data that states did provide, they reached five main conclusions. These conclusions are: (1) In most states with three or more years of comparable test data, student achievement in reading and math has gone up since 2002, the year NCLB was enacted; (2) There is more evidence of achievement gaps between groups of students narrowing since 2002 than of gaps widening. Still, the magnitude of the gaps is often substantial; (3) In 9 of the 13 states with sufficient data to determine pre- and post-NCLB trends, average yearly gains in test scores were greater after NCLB took effect than before; (4) It is very difficult, if not impossible, to determine the extent to which these trends in test results have occurred "because" of NCLB. Since 2002, states, school districts, and schools have simultaneously implemented many different but interconnected policies to raise achievement; and (5) Although NCLB emphasizes public reporting of state test data, the data necessary to reach definitive conclusions about achievement were sometimes hard to find or unavailable, or had holes or discrepancies. More attention should be given to issues of the quality and transparency of state test data. Study methods are appended. Glossary of Technical Terms is included. (Contains 5 boxes, 6 figures, 22 tables and 5 footnotes.)