This dissertation researched the scope of mathematics achievement disparities in the United States and how these disparities can be minimized through a self-efficacy lens. To answer pertinent research questions with data from the National Assessment of Educational Progress (NAEP) 2019 mathematics assessment in Grades 4, 8, and 12, gap analyses, two-level cross-sectional multilevel modeling, and two-level structural equation modeling were conducted. The discoveries demonstrated that the excellence gaps in U.S. in mathematics achievement by race/ethnicity, gender, NSLP, ELL, and IEP persisted and widened from Grade 4 to Grade 8 yet decreased at Grade 12. Self-efficacy was a noteworthy predictor of students' mathematics achievement, displaying large effect sizes across grades. The disparities in mathematics achievement by student subgroups lessened when students' self-efficacy was equal. Moreover, self-efficacy mediated the relationships between mastery-approach goals, performance-approach goals, interest, persistence in learning and achievement. Additional results demonstrated that the status of being racially/ethnically underrepresented students partially moderated the connections between motivational variables and mathematics achievement. School locale and the percentage of underrepresented students significantly impacted students' achievement. This dissertation underscores the importance of self-efficacy in closing mathematics achievement gaps and improving students' mathematics achievement. [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.]