Student foundational knowledge of science, technology, engineering, and mathematics (STEM) is formed in their elementary education. Paradoxically, many elementary teachers have constrained background knowledge, confidence, and efficacy for teaching STEM that may hamper student STEM learning. The association between teacher preparation to teach STEM and student achievement in STEM motivated the authors' professional development program. The authors created and implemented a professional development program to address K-5 teacher confidence for, attitudes toward, knowledge of, and efficacy for teaching inquiry-based STEM. Using data from 2 independent cohorts the authors found significant and consistent increases in pre- to postinstitute assessments of teacher confidence, efficacy, and perceptions of STEM. Further, they found increased participant attention toward linking STEM curriculum and instruction to learning standards. Implications and suggestions for future research are discussed. (Contains 3 tables.)