This research study focused on the effects of departmentalization in the elementary school setting. Previous research on the topic of departmentalization in elementary school settings presented inconclusive results. The purpose of this study was to collect and analyze data from self-contained and departmentalized classrooms in fourth and fifth grade mathematics and reading at a preschool through eighth grade (P-8) school district in the southwestern portion of Cook County, Illinois to evaluate the effectiveness of departmentalization in the elementary setting. To conduct this study, the researcher collected academic achievement data from the i-Ready Diagnostic Assessment and the Illinois Assessment of Readiness. On the i-Ready Diagnostic Assessment, the data were found to be statistically significant in mathematics and reading, so the null hypotheses were rejected. The effect sizes showed that self-contained classrooms had a moderate effect size on academic achievement in both subject areas. On the Illinois Assessment of Readiness, the data were found to be statistically non-significant in both mathematics and reading, so the null hypotheses were accepted. These mixed results led the researcher to conclude that other factors, such as teacher quality, may influence student achievement more than the type of classroom organizational structure. [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.]