How should teachers spend their STEM-focused professional learning time? To answer this question, we analyzed a recent wave of rigorous new studies of STEM instructional improvement programs. We found that programs work best when focused on building knowledge teachers can use during instruction: knowledge of the curriculum materials they will use, knowledge of content and how content can be represented for learners, and knowledge of how students learn that content. We argue that such learning opportunities improve teachers' professional knowledge and skill, potentially by supporting teachers in making more informed in-the-moment instructional decisions.