The purpose of this study was to: (1) describe elementary teachers' perceptions on their preparedness to implement the English Language Arts Common Core State Standards (ELA-CCSS); (2) determine how perceptions influenced changes in instructional practices; and (3) to explore ELA-CCSS implementation challenges and/or barriers in supporting teacher development. This study was conducted using a multi-methods research design involving quantitative data from a survey, and qualitative data from interviews. The conceptual framework for this study centered around a history of the Common Core State Standards, education reform implementation research, understandings about the Common Core State Standards, and reconstructing teacher practice. Results revealed that professional development effect on instructional practice was limited; preparedness was a learning process; a mismatch existed between preparedness and instructional practices in relation to teacher change; and supports offered in district were not strong enough to advance reform implementation. Districts, administrators, and teachers are encouraged to integrate teacher participation in decision-making processes; develop a robust professional learning community model; ensure alignment of teacher supports with actual teachers' needs, and augment teacher agency as a barrier to implementation passivity. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]