This study was conducted in response to the growing concern about the lack of U.S. students majoring in STEM fields and pursuing STEM careers (NSF, 2013). In order for the U.S. to compete in a global economy that is increasingly technologically-based, a skilled STEM workforce is a necessity (National Academies, 2010). Understanding the factors that encourage confidence in science and intent to pursue science-related careers remains a national mandate. Using data from a nationally representative sample of 21,440 students from 940 schools, the High School Longitudinal Study of 2009 (HSLS:09), this study provides insight into the factors that predict science self-efficacy and STEM career intent. Guided by Lent, Brown & Hackett's (1994) Social Cognitive Career Theory (SCCT), both student-level and school-level factors were examined. The findings indicate that the personal inputs of being African American, female, and having a science identity predict both science self-efficacy and STEM career intent. Participating in no extracurricular informal learning experiences predicts both science self-efficacy and STEM career intent, while participating in more than one extracurricular learning experience only predicts self-efficacy. Background contextual affordances such as socioeconomic status had no relationship to either science self-efficacy or STEM career intent. Parents who were very confident and somewhat confident in helping their child with science homework predicted science self-efficacy but showed no relationship to STEM career intent. None of the school-level variables predicted science self-efficacy, but the percentage of students on free/reduced lunch, students in schools located in the city, and students in Catholic schools predicted STEM career intent. Finally, the relationship between contextual affordances (SES and parental support) and science self-efficacy and STEM career intent did vary by sex for both outcome variables regarding socioeconomic status, but not regarding parental support. Several implications arose from this study. First, science identity is a very useful construct that can be used to further examine science self-efficacy and STEM career intent. Based on this study's findings, educators and policy makers should seek ways to help foster student science identity. Second, efforts to encourage girls in STEM have not been successful on a large scale and more strategies need to be explored. Third, this study makes the case for continued involvement in informal science experiences that provide students a chance to explore science at their own pace in a non-evaluative environment. Finally, while race/ethnicity has traditionally been a predictor of science self-efficacy and STEM career intent, this study suggests that there may be subcategories within race/ethnicity such as cultural perspective and additional constructs such as critical race theories that may shed light on the nuances at play, especially in the case of students who self-identify as African American. [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.]