As of 2008, approximately 40% of eighth grade students in the United States enroll in Algebra I (National Council of Teachers of Mathematics, 2008). Although research has shown that students have more opportunities to take advanced mathematics courses by taking eighth grade Algebra I, in the United States, approximately only one-third to one-half of eighth grade Algebra I students take a mathematics course beyond Algebra II. This mixed-methods study investigates the implications of eighth grade Algebra I on high school mathematics achievement--the mathematics courses students take in ninth through twelfth grades, the reasons for students' mathematics course-taking decisions, and grades earned in mathematics courses from eighth through twelfth grades. Academic data were collected on 1,595 eighth grade Algebra I students from a large school district in Texas from the graduating classes of 2009-2012. In addition to analysis of archival data, data were collected and analyzed based on six focus groups held with 2011-2012 twelfth grade students who had taken eighth grade Algebra I. These students were grouped according to three criteria: enrolled in Calculus, enrolled in a mathematics course lower than Calculus, and not enrolled in a mathematics course. Data were analyzed using statistical techniques including: grounded theory, descriptive analyses, structural path analysis with conditional probabilities, discriminant function analyses, two-way contingency table analyses, Chi-square tests of independence, and linear regression analyses. Results show the most common courses taken by eighth grade Algebra I students are: Geometry PAP, Algebra II PAP, Pre-Calculus PAP, and AP Calculus AB. Although the state of Texas' 4x4 graduation requirement has not made an impact on enrollment in Calculus, the data show it is a significant predictor in students taking mathematics and, specifically, advanced mathematics courses in twelfth grade. Grades earned by students in eighth grade Algebra I were also found to be significantly related to high school mathematics grade averages. The most important finding in the study is the emergence of a new theoretical framework rooted in cultural capital, social capital, and self-efficacy--mathematics capital. Mathematics capital is defined as accumulated profits for students who enroll and complete advanced mathematics courses that can transcend to other forms of capital. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]