Young children are able to benefit from early science teaching but many preschool teachers have not had opportunities to deepen their own understanding of science or to develop their pedagogical content knowledge (PCK) in relation to specific science topics and concepts. This study presents the results of efficacy research on Foundations of Science Literacy (FSL), a comprehensive professional development program designed to support teachers' knowledge of early childhood science; their PCK around 2 physical science topics (water, and balls and ramps); and their abilities to plan, facilitate, and assess young children's learning during inquiry-based science explorations. Research Findings: In a randomized trial with 142 preschool teachers and 1,004 4-year-old children, FSL teachers demonstrated significantly higher quality science teaching in general and greater PCK in the 2 physical science topics than did teachers in comparison classrooms. Furthermore, children in FSL classrooms performed significantly better than children in comparison classrooms on tasks involving floating and sinking, and an instrumental variable analysis suggests that the quality of classroom science instruction mediated the relationship between teacher participation in FSL and student outcomes. Practice or Policy: Findings support the use of comprehensive early science professional development programs designed to bolster teacher knowledge and PCK.