The purpose of this study was to explore the differences between Reading/Language achievement gains of students in multi-age classrooms to the Reading/Language achievement gains of their peers in traditional, single-age classrooms. The causal-comparative quantitative approach to exploring cause-and-effect relationships was employed in this study. In this study, the effect of multi-age grouping and single age grouping was analyzed and compared using TCAP Reading/Language raw gain scores. Raw gain scores were used to determine the amount of progress children make from one year to the next regardless of their level of achievement. Findings in this study were mixed. Some significant differences were found in favor of single-age classrooms. However, the calculation of effect size showed no practical significance. Significance was also revealed in favor of males over females in both single-age and multi-age classrooms; although, effect size indicated only a small to moderate practical significance exists. This study provides an overview of the history of American educational structures. It might be helpful for the educational community in evaluating one dimension of the effectiveness of multi-age groupings. Teachers and administrators could benefit from the comparisons made in this study and as a result make better decisions regarding the delivery of instruction and the structuring of school classrooms. [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.]