Policymakers have sought to increase the rigor of content standards since the 1990s. However, the literature examining the effects of reforms to content standards on student outcomes is still developing. This study examines the extent to which the Common Core State Content Standards (CC) affected student achievement and the size of achievement gaps. To identify the effect of CC I compare early implementors of the CC to late implementors of the CC in a Difference-in-Differences framework. I conducted a document analysis to measure preparation for and implementation of the CC standards, which I merge together with the National Assessment of Educational Progress student-level data. I then exploit variation in the timing of state implementation of the CC to identify its effect on students overall and academically vulnerable groups. I find that the CC has a positive effect on math scores in 4th and 8th grade, but not in reading. The CC had a large positive effect on economically advantaged students, but a null effect for economically disadvantaged students. Demanding better results without addressing the structural issues burdening economically disadvantaged students may result in unintended consequences.