The purpose of this study is to investigate how high school academic and non-academic factors are related to college readiness and performance in freshman-level mathematics and science courses and retention in STEM fields. High school academic indicators for college readiness will include performance in high school science and mathematics courses, high school GPA, SAT or ACT scores, high school accelerated college credit programs. Additionally, a secondary purpose will be to identify the non-academic factor at the secondary school level that best predicts students' chances of success and persistence in college freshman STEM major courses. Data from 768 first-time freshman college students in the fall semester of 2018 who were enrolled in STEM majors were used for the study sample. This study's findings are intended to improve an understanding of the relationship between high school students' critical academic and demographic variables and success in freshman-level college mathematics and science courses. Based on these findings, guidance counselors, science educators, and higher education institutions can develop a framework to determine meaningful measures to prepare students for rigorous college courses and advise them to improve much-needed skills to succeed in college. The transition from high school to college comes with social and educational challenges for many students, and these challenges are especially difficult for economically and educationally disadvantaged students. This study's results provided data useful to similar college settings in developing effective interventions to increase first-year persistence. This study's findings suggested that High school GPA, SAT score, and AP courses appeared to correlate positively with most outcome variables. AP courses as an accelerated college credit program were significant variables in predicting cumulative credits earned towards degree completion and the strongest significant predictor of college biology performance among all academic variables. High school GPA accounted for a higher cumulative GPA by the end of freshman semesters, was a strong predictor of cumulative GPA earned through the 2018 fall and 2019 spring semesters with or without controlling demographic variables. Moreover, HS GPA was the most significant variable to predict retention among three variables, including AP credits, HS GPA, and gender, significant for most dependent variables. Furthermore, HS GPA was a significant predictor of college mathematics with or without controlling demographic variables. SAT composite score was the strongest predictor of college chemistry among all the academic variables. Lastly, gender, among demographic variables, significantly correlated with college chemistry and introductory college mathematics, but not on college biology, college mathematics, and engineering. This study contributes to the existing literature on factors that influence freshman college mathematics and science courses' performances. However, this study had its limitations, and the results cannot be generalized because of its small sample size and sampling method, which was not either through the random, stratified, cluster, or other recognized sampling methods. Preparing students for College is a multifaceted and complicated task. This exploratory study will be helpful as a guide for further research. [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.]