This dissertation investigates the social determinants of STEM curriculum tracking and changes in the nature of STEM tracking in both US and cross-national settings over recent decades in five NCES High School longitudinal datasets and in seven TIMSS studies. Methodologically, this dissertation contributes to the field by developing a consistent and comprehensive transcript-based and instructional content-based measurement scheme of the most important organizational dimensions of tracking. This research also advances the current state of knowledge in the field of STEM curriculum and education inequality by adding rich description of the organizational dimensions of tracking both in the US and globally and drawing attention to the ways in which STEM curriculum tracking departs from its functional ideal. Although tracking systems are intended to benefit all students, too often they result in widening educational inequality. Using rich data on student course taking over the course of secondary education, I produce new measures at both the school-level and country-level capturing inequality in STEM opportunity to learn, both of which are essential to the STEM pipeline. At the same time, as with prior policy-focused studies, this research project draws attention to tracking policies and practices themselves in hopes of generating organizational awareness of, and introspection by administrators towards tracking. In an era of curriculum intensification, this work helps researchers and policymakers understand how schools with different compositional characteristics expose students to differently tracked learning environments. Moreover, this dissertation study also advocates a systematic understanding of tracing the fundamental sources of inequality in opportunity to learn. Overall, while it may be difficult to describe a given determinant of tracking in the context of the secondary data analyses in this dissertation as unequivocally dysfunctional, this research may nevertheless encourage scrutiny of curricular policy and practice that too often go unexamined. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]