Building on Social Cognitive Career Theory and Xueli Wang's conceptual model of STEM choice, this study advances and tests a model seeking to understand STEM readiness and intention to pursue STEM fields among a representative national sample of 9th grade students drawn from the High School Longitudinal Study of 2009. Structural equation model results suggest several contextual influences (e.g., personal characteristics) and social-cognitive variables (e.g., math and science self-efficacy) contributing to building STEM readiness and students' intention to major in STEM during high school. In general, STEM readiness is impacted directly by SES, math ability, parental involvement, math self-efficacy, science self-efficacy, math interest, and science interest. Intention to major in STEM is directly impacted by STEM readiness, as well as high school students' interest in math and interest in science. Overall, this study expands our knowledge of the process that leads high school students to become prepared for and aspire to pursue majors in STEM.