The purpose of this mixed-methods study was to first determine whether literacy performance changed over time for third and fourth graders of a private school in Tennessee with the implementation of the Units of Study co-authored by Lucy Calkins and Teachers College Reading and Writing Project (UOS). The researchers utilized data from the school's annual Educational Records Bureau Comprehensive Testing Program achievement scores from 2012 through 2021. The researchers also determined if there was a predictive relationship between the number of years of implementation of the UOS and the literacy achievement scores. A questionnaire was administered, and interviews were conducted to collect teachers' and administrators' perceptions on the challenges and successes of the implementation of the UOS and their perceptions of the impact of the UOS on students' literacy achievement. The qualitative data revealed the themes of a strong curriculum, variety of materials, structured lessons, time constraint with the UOS, disengaging lessons, density of the UOS curriculum, the influence of social interactions on literacy performance, and the UOS' impact on different types of learners that included three subthemes: low-level learners, high-level learners, and how previous knowledge and experience influenced students' learning within the UOS. Quantitative findings revealed that literacy performance did significantly change over time. It was also revealed that a negative predictive relationship existed between the number of years of implementation of the UOS and literacy achievement scores. [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.]