This sequential explanatory mixed methods study examines the impact of the Language Essentials for Teachers of Reading and Spelling (LETRS) professional learning intervention on teacher knowledge within the context of South Carolina's reading policy. Quantitative analyses of pre- and post-assessment data indicate significant improvements in teacher content knowledge of foundational literacy skills following LETRS training. Descriptive statistics and paired-samples t-tests demonstrate substantial gains in knowledge, particularly in Volume 1 of the LETRS curriculum. The study reveals notable increases in pre- and post-assessment scores, suggesting the effectiveness of LETRS in enhancing teacher expertise. Qualitative findings from focus groups highlight the positive impact of LETRS on teacher knowledge and classroom practices. Stakeholders at various levels identify LETRS as a valuable tool for improving literacy instruction, citing enhanced understanding of foundational reading skills and observable improvements in student performance. However, challenges such as the need for structured time and aligned instructional resources are identified as inhibiting factors. Leadership emerges as a critical factor in successful LETRS adoption. Effective leaders prioritize LETRS training, create supportive environments for teachers, and ensure alignment with best practices in literacy instruction. Policy implications highlight the importance of prioritizing the science of reading in state initiatives, addressing implementation challenges, and integrating LETRS into teacher and education leader preparation programs. Practical recommendations include establishing support networks for administrators, providing dedicated time for LETRS learning, and ensuring alignment of instructional resources with LETRS principles. Future research should explore the relationship between LETRS training and classroom practice, investigate its impact on student outcomes, and contribute to the discourse on statewide implementation efforts. Overall, this study provides valuable insights into the role of LETRS in enhancing teacher knowledge and advancing literacy education in South Carolina. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]