The development of state implementation plans (SIPs) for attainment of criteria pollutant standards is an integral component of air quality management in the United States. However, the content and efficacy of SIPs have rarely been examined systematically. Here, 20 SIPs developed in response to the 1997 8-hr ozone standard are reviewed as case studies of attainment efforts at the state level.Comparison of observed and model predicted ozone concentrations shows the U.S. Environmental Protection Agency (EPA) recommended modeled attainment test to be a somewhat conservative predictor of attainment. Among 12 SIPs for regions that sought attainment by 2009, the test correctly predicted attainment and nonattainment in four and five regions, respectively; in the other three regions, attainment was observed despite predictions of nonattainment. However, weight-of-evidence determinations and deviations from the recommended modeled attainment test methodology led five of these SIPs to predict attainment that was not in fact observed by 2009; three of those regions achieved attainment in 2010. Ozone and NO 2 concentrations declined across much of the United States during the period covered by the SIPs, with rates of improvement strongly correlated with the initial pollution levels and hence greatest in nonattainment regions. However, at monitors with mid-range levels of ozone initially, rates of reduction were largely independent of the initial attainment status of the region. This is consistent with the fact that apart from California, the majority of ozone precursor reductions documented by SIPs resulted from federal measures rather than from state or local controls specific to the nonattainment regions.Implications: At a time of sharp debate in the United States about strengthening ambient ozone standards, this study highlights strengths and weaknesses of recent state implementation plans. Substantial improvements in air quality have been observed, bringing the majority of regions into attainment of the 1997 ozone standards. The extent to which state-level measures contributed to those improvements is unclear. The standard modeled attainment tests provided somewhat conservative predictions of future attainment, but alternate approaches predicted attainment that was not in fact observed. Greater caution may be needed in weight-of-evidence analyses to avoid overly optimistic predictions of future attainment.