In this chapter, we first synthesize the last two decades of research on students’ intentions, choices, and persistence in the STEM pipeline (i.e., earning a postsecondary degree in STEM and/or joining the STEM workforce). In doing so, we frame the literature review by the social cognitive career theory (SCCT; Lent, Brown, & Hackett, 1994) with a special focus on underrepresented minorities (URMs) in STEM (Yu, Corkin, & Martin, 2017). SCCT centralizes three variables in forming career decisions: personal inputs, motivational factors, and contextual factors. We then introduce a 4-year longitudinal study tracking STEM intentions of high school students. The longitudinal study started with the 9th grade students and will conclude when the same students are in the 12th grade. As the study is in its third year, findings from the first and second year are summarized here. From 9th grade to 10th grade, there are positive changers (i.e., changing intention from non-STEM to STEM college major), negative changers (i.e., changing intention from STEM to non-STEM college), and non-changers. Focusing on negative and positive changers, findings from data collected through interviews of students delving into reasons for changing intentions from 9th to 10th grade are presented, describing the most critical factors influencing students’ decision making/changing. We discuss the results, implications, and limitations of the study informing both the research field (e.g., study design, future research areas) and the practice (e.g., focus on school level factors and steps to improve individual motivation).