This dissertation aims to advance our understanding of Asian American students' choices and trajectories in Science, Technology, Engineering, and Mathematics (STEM) fields, especially in math-intensive STEM sub-fields, by investigating Asian American female and male students' math-related motivational beliefs held in high school and whether/how these motivational beliefs are associated with their intended major choices in college. Specifically, using a nationally representative dataset, the first analytic chapter of this dissertation offers a descriptive picture of three aspects of Asian American female/male high school students' math-related motivational beliefs (i.e., math self-efficacy, math interest, and math utility value), by disaggregating them into five Asian ethnic subgroups. Disparities in high school math performance and similarities in math-related motivational beliefs among Asian American students from different ethnic subgroups are revealed. Additionally, a mismatch between East Asian American female students' high math achievement and relatively low motivational beliefs in math (after family socioeconomic status, generational status, and prior math performance are adjusted for) is observed. The second analytic chapter provides a rich descriptive picture of Asian American female/male high school students' math-related motivational beliefs, in comparison with those held by their peers from other racial/ethnic backgrounds. The researcher finds that although Asian American students gained the highest level of math performance in 9th grade, they do not necessarily report higher levels of math-related motivational beliefs than students in other racial/ethnic groups do. In fact, net of family socioeconomic status, generational status, and prior math performance, Asian American male and female students report significantly lower levels of math self-efficacy than their African American/ Black counterparts. The third analytic chapter explores whether motivational beliefs toward math held in high school are associated with White and Asian American female/male students' subsequent college major choices. Findings from the third chapter reveal that high school motivational beliefs in math are significant and positive predictors of White male, White female, and Asian American male students' intended college major choices in math-intensive STEM sub-fields (vs. non-STEM fields), but are not associated with Asian American female students' choices of a math-intensive STEM major (vs. a non-STEM major) in college. [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.]