This research study sought to determine the effects of Flipped Classroom Instruction (FCI) on middle-grade students' mathematics achievement. In an FCI environment, students are initially exposed to new content outside of the classroom, through screencasts and assigned readings; class time is reserved for answering questions and engaging students in relevant activities for assimilating the new content. The current study compared the impacts of (1) traditional instruction and (2) flipped classroom instruction on students' mathematics achievement. Participants in this study were middle grade students (n = 34) in grades 7 through 9 from a typical private school in a Northeastern region of the United States (U.S.). All participating students had already received traditional instruction in mathematics during the 2019-2020 academic year. Their final assessments were designated as pretests. During the 2020-2021 academic year, the same students received mathematics instruction via FCI, and their final assessments were designated as posttests. A paired-samples t-test revealed a significant (p < 0.05) improvement in students' mathematics achievement levels following their FCI experience. Additionally, regression analysis revealed a strong correlation (R = 0.89) between students' pretest and posttest scores, resulting in a significant prediction equation (p < 0.001) of students' mathematics achievement via FCI. An ANCOVA revealed no significant difference in the mathematics achievement levels of males and females. Further analyses, using multiple regression, reduced concerns for potential confounding effects of students' attitudes and motivations on their mathematics achievement. It was recommended that future research examine the impacts of FCI on students' learning independence and problem-solving skills. [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.]