This study examines the impact of high-stakes school accountability, capacity, and resources under NCLB on reading and math achievement outcomes through comparative interrupted time-series analyses of 1990-2009 NAEP state assessment data. Through hierarchical linear modeling latent variable regression with inverse probability of treatment weighting, the study addresses pre-NCLB differences in state characteristics and trends to account for variations in post-NCLB gains. While the states' progress was uneven among different grades, subjects, and subgroups, NCLB did not yet evidence sustainable and generalizable high-stakes accountability policy effects. Improving average achievement as well as narrowing achievement gaps was associated with long-term statewide instructional capacity and teacher resources rather than short-term NCLB implementation fidelity, rigor of standards, and state agency's capacity for data tracking and intervention. (Contains 1 figure, 3 tables, and 12 notes.)