This study, "Teacher Insight: The Implementation of the Common Core State Standards in California School Districts" was designed to glean teacher voice on the large-scale reform. With a need for a global workforce in a 21st century society the existing education system is undergoing a tremendous change in order to prepare students for college and career. The purpose of this sequential explanatory mixed methods study was fourfold: (a) to determine how the implementation of the Common Core State Standards is changing teacher practices related to curriculum, instruction, and assessment; (b) to investigate the concerns and challenges faced by teachers as they implement the Common Core State Standards; (c) to determine what types of professional development teachers have been offered regarding the Common Core State Standards and what they perceive has been most beneficial; and (d) to determine what teachers still need from their site leadership to make the implementation successful. Additionally, this study was performed with surveys and interviews administered in three California school districts. Upon examination of the responses from teachers, this study yielded four conclusions. First, implementation of the Common Core State Standards has changed teacher instructional practices. With the fusion of the 21st century skills into the Common Core State Standards teachers are challenged with teaching the new academic standards and simultaneously providing instruction with 21st century skills. Second, implementation of the Common Core State Standards has generated concerns and challenges of teachers. Third, teachers state that collaboration is the most beneficial form of professional development. Fourth, the implementation has created needs from teachers of site leadership. The study also yielded four recommendations. First, it is recommended that school districts provide teachers with professional development on 21st century skills. Second, it is recommended that school districts allocate funding for the purchase of Common Core State Standard resources that have both rigor and relevance. Third, it is recommended that collaboration be the type of professional development that districts utilize, as teachers state it is the most beneficial. Fourth, site leaders are advised to address the needs of teachers by providing them with resources they need for the implementation. [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.]