How should teachers spend their STEM-focused professional learning time? To answer this question, Heather Hill, Kathleen Lynch, Kathryn Gonzalez, and Cynthia Pollard analyzed a recent wave of rigorous new studies of STEM instructional improvement programs. They found that programs work best when focused on building knowledge teachers can use during instruction. This includes knowledge of the curriculum materials they will use, knowledge of content, and knowledge of how students learn that content. They argue that such learning opportunities improve teachers' professional knowledge and skill, potentially by supporting teachers in making more informed in-the-moment instructional decisions.