Researchers and policymakers are interested in the pathway to the STEM workforce given projections about a workforce shortage and the underrepresentation of women and people of color in STEM. These examinations often rely on the STEM pipeline model as a framework for understanding STEM persistence, which uses a STEM degree as a proxy for future workforce entry. However, this approach limits knowledge on STEM persistence to students' postsecondary years and is not an appropriate framework for examining persistence from a longitudinal perspective. Few studies use longitudinal data and methods appropriate for examining STEM persistence and identifying when attrition from the pathway to the workforce is likely to occur. I used STEM career aspirations and social cognitive career theory as a guiding framework to track students on their trajectory to the STEM workforce. Using data from the Education Longitudinal Study of 2002 (ELS:2002), I examined the career aspirations histories from grade 10 to age 26 of high school students with early STEM career aspirations. I constructed alluvial diagrams to explore patterns of differential change in students' career aspirations over time and how these patterns relate to STEM-related milestones. I used survival analysis to determine whether and when students lose their STEM career aspirations for the first time during their secondary, postsecondary, and early adult years. I applied discrete-time hazard modeling to determine how students' characteristics, background affordance, and math self-efficacy contribute to their likelihood of no longer aspiring to a STEM career. I found that students' career aspirations are unstable over time. Nearly half of the students in the sample lose their STEM career aspirations by grade 12. Still, it was more likely that students who reached STEM-related milestones aspired to a STEM career at the juncture preceding those achievements. While students' early STEM career aspirations did not appear to have a considerable impact on reaching STEM-related milestones, most of the students who reached those milestones persisted in their grade 10 STEM career aspirations. Students' gender, race, parental educational expectations, math achievement, and math self-efficacy all have statistically significant impacts on the likelihood of no longer aspiring to a STEM career. I provide implications for future research, policy, and practice related to STEM persistence. [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.]