Background: As of 2016, 42 states and the District of Columbia have adopted the Common Core State Standards (CCSS). Tens of millions of students across the country completed high school before their schools were able to fully implement the CCSS. As with previous standards-based reforms, the transition to the CCSS-aligned state education standards has been accompanied by curriculum framework revisions, student assessment redesigns, and school accountability and educator evaluation system overhauls. Purpose: Even if the new standards may improve student learning once they are fully implemented, the multitude of changes at the early implementation stage of the reform might disrupt student learning in the short run as teachers, schools, and communities acclimate to the new expectations and demands. The goal of this study is not to evaluate the merits and deficiencies of the CCSS per se, but rather to investigate whether college readiness improved among high school students affected by the early stages of the CCSS implementation, and whether students from different backgrounds and types of high schools were affected differently. Research Design: We focus on three cohorts of eighth-grade students in Kentucky and follow them until the end of the 11th-grade, when they took the state mandatory ACT tests. The three successive cohorts--enrolled in the 8th-grade between 2008 and 2010--each experienced different levels of exposure to CCSS transition. Using ACT scores as proxy measures of college readiness, we estimate cohort fixed-effects models to investigate the transitional impact of standards reform on student performance on the ACT. To gauge the extent to which the implementation of CCSS is directly responsible for any estimated cross-cohort differences in student ACT performance, we conduct additional difference-in-differences analyses and a falsification test. Data: Our data include the population of three cohorts of 8th-graders enrolled in Kentucky public schools between 2008 and 2010. The total analytic sample size is 100,212. The data include student test scores, student background characteristics, and school characteristics. Findings: In the case of the CCSS transition in Kentucky, our findings suggest that students continued to improve their college-readiness, as measured by ACT scores, during the early stages of CCSS implementation. Furthermore, evidence suggests that the positive gains students made during this period accrue to students in both high- and low-poverty schools. However, it is not conclusive that the progress made in student college-readiness is necessarily attributable to the new content standards. Conclusions: As we seek to improve the education of our children through reforms and innovations, policymakers should be mindful about the potential risks of excessive changes. Transition issues during the early stages of major educational changes sometimes lead to short-term effects that are not necessarily indicative of the longer-term effects of a program or intervention. Nevertheless, standards-based reforms are fairly frequent, and each takes multiple years to be fully implemented, affecting millions of students. Therefore, we encourage researchers and policymakers to pay more attention to the importance of transitional impact of educational reforms.