The Common Core State Standards (CCSS) for social studies are based on critical thinking and literacy skills. These new mandates are expected to lead to curricular and instructional changes within social studies classes. This qualitative study explored how the CCSS might have impacted the curricular and instructional decision-making of middle school social studies teachers and ultimately how the CCSS might affect a teacher’s gatekeeping role. As the CCSS initiative is fairly new, there is little research on the instructional practices being used to support the needs of teachers implementing these new standards in their classrooms as well as the processes, challenges, and successes teachers experience in addressing the CCSS in their classrooms. This study fills the gap of information lodged between a policy mandate and implementation in the classroom by contributing to the literature in the area of social studies education and the types of instruction social studies teachers may use to achieve the goals within the CCSS. Data gleaned from this study demonstrates that the CCSS had an influence on teachers’ instructional and curricular decision-making. CCSS influenced teachers’ decision-making in three domains: teacher beliefs ((individual teacher’s beliefs regarding the CCSS, including his or her personal beliefs regarding the CCSS and self-confidence to teach the skills associated with the CCSS), student assessment (the connection between standardized assessments and the CSSS), and best practices (recommended best practices by CCSS that were already being used in the classroom). As a result, teachers increased the number of the types of instructional strategies that focused on the critical thinking skills advocated by CCSS such as analyzing primary and secondary sources and using evidence from multiple sources to complete a Document Based Question (DBQ). The study also revealed that teachers felt inadequately prepared to fully implement the CCSS in their classrooms due to insufficient teacher education geared to CCSS, resources, and inconsistencies of the focus of the CCSS within participants’ Professional Learning Communities.