The authors' goals in this study are to use both the National Assessment of Educational Progress (NAEP) and state accountability test score data to (1) provide a detailed description of the magnitude and trends of state-level academic achievement gaps among cohorts of students entering school in the 1990s and 2000s; (2) investigate the extent to which patterns and trends in gaps vary among states; and (3) provide preliminary evidence regarding the impact of NCLB (No Child Left Behind) on achievement gaps. The authors use Main NAEP test score data from 4th- and 8th-graders in 1990-2009. They also use state-level categorical proficiency count data for students in grades 3-8 in 2001-2010, collected with the help of state department of education officials, as well as the Office of Planning, Evaluation, and Policy Development at the U.S. Department of Education. The authors are primarily interested in white-black and white-Hispanic gaps, though they also report female-male achievement gaps. The authors' findings to date indicate, first, that black-white and Hispanic-white achievement gaps have narrowed in the last decade or more. Male-female gaps appear largely unchanged over the same time period. Second, there is considerable variation across states in both the magnitude and trends in achievement gaps. Third, the patterns evident so far do not suggest a strong effect of NCLB on achievement gaps, though these analyses are not yet complete. (Contains 2 tables and 3 figures.)