The purpose of this study was to assess the impact of the Pathways to STEM Initiative (PSI) on students and science teachers and to describe the level of PSI implementation. One group of middle schools participated in PSI, which included project-based science, technology, engineering, and math (STEM) coursework; extra-curricular STEM opportunities for students; and teacher professional development. A multivariate matching algorithm was used to identify a comparison group of schools that received the participating district's standard science curriculum. The students in the study schools were 62% Hispanic/Latino, 17% Black/African American, and 12% White. Additionally, 23% of the students were English language learners. The study compared students' science achievement and teachers' beliefs about science and attitudes toward STEM across the treatment and comparison schools and assessed the fidelity of implementation of critical program components. Student impact analyses indicated that participation in PSI for one year for 6th graders and two years for 7th and 8th graders did not improve students' science achievement. Teacher impact analyses did not show that PSI had an effect on the science teachers' beliefs about science and attitudes toward STEM. Results from the implementation study did not reveal consistently high levels of implementation of PSI in the four participating middle schools. PSI could have a positive impact on students and teachers in different settings and/or when the intervention is implemented with higher fidelity to the program model. Given these possibilities, further research on PSI is warranted. The following are appended: (1) Pathways to STEM Initiative (PSI) Logic Model; (2) Comparison School Selection; (3) Baseline Balance Testing; (4) Impact Analyses; and (5) Implementation Findings from Year 1.