This study aimed to investigate the impact of LETRS professional development on reading proficiency. I used ANOVA tests to analyze third- through fifth-grade EOG assessment scores for changes in proficiency from 2021-2023. The results indicated a statistically significant change in the means for all grades. Third-grade results reflected the most impact from the professional development. The analysis also showed that the third-grade achievement gap had declined slightly. A dependent t test was conducted to ascertain any change in the growth index scores of schools. The test results showed no statistically significant change. The study's limitations include the lingering impact of the COVID-19 pandemic. The study could not show how much or how little the changes in instruction due to the pandemic influenced the results. I attributed the increase in proficiency to the professional development being grounded in the Cognitive Foundations of Reading Framework. Stakeholders should continue to invest in professional development that is based on reading research. [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.]