Two studies investigated the effect of departmentalized instruction on ELA and math achievement among 5th grade students in a large, urban school district. In 84% of the findings from the two studies combined, students in departmentalized classrooms produced significantly higher scores in ELA and mathematics, relative to students in generalized classrooms. In ELA, significantly higher scores were obtained in departmentalized classrooms across genders among general-education students, special-education students, and English language learners. In mathematics, the results were inconsistent, showing advantages across genders in departmentalized classrooms for general-education students and special-education students, and advantages in generalized classrooms for English language learners. In the main effect, the results support use of departmentalized instruction in upper-elementary grades. Future research should explore whether an increased number of standards can be addressed in the departmentalized model, the relationship between English as a new language support and departmentalized math instruction, the impact of the model on student-teacher rapport, and the best paths to implement the model. This research encourages schools to consider the departmentalized model to increase student achievement and reduce teacher stress and attrition, and it supports changes to teacher certification policies. [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.]