This study is designed to assess the effects of states' adoption of college- and career-ready (CCR) standards on student achievement. Relying on state-level NAEP data from 1990 to 2015, we use comparative interrupted time series analyses to assess whether the adoption of CCR standards led to a larger improvement in student achievement in states with lower prior proficiency standards (where the CCR standards represent a stronger form of "treatment") than in states with higher prior standards. Preliminary results show that adopting CCR standards had a small positive effect in grade 8 reading, but not in grade 4 reading or in math. Findings from this study will provide much needed information about the current wave of standards-based reform.