The purpose of this paper is to describe teacher outcomes from participating in an intensive science, technology, engineering, and mathematics (STEM) professional learning (PL) initiative for middle school science teachers in the United States. The initiative included intensive summer coursework and ongoing support (e.g., individual coaching, professional learning communities), and focused on enhancing teachers' STEM instruction and their pedagogical content knowledge (PCK), with the ultimate goal of improving student outcomes. In this mixed method study, we examine change across time in teachers' beliefs, use of STEM instruction, and PCK. In general, we did not observe statistically significant change in teachers' beliefs, use of STEM instruction, or PCK from the beginning to the end of Year 1. In follow-up focus groups, ten teachers described their perceptions of student outcomes of STEM instruction and identified tensions related to implementation. We offer implications intended to inform future work in the area of STEM PL.