To develop Science, Technology, Engineering, and Mathematics (STEM) talents, both researchers and policy developers recommend that educators begin early. In this randomized study, we document the efficacy of teacher professional development and a rich problem-based inquiry curriculum to develop the science talent of elementary students. The intervention, STEM Starters, a federally funded Jacob K. Javits project, provided sustained and embedded professional development to classroom teachers and to pull-out gifted program teachers to support the implementation of a problem-based curriculum in their classrooms. During the intervention, randomly assigned teachers participated in 120 hr of professional development that focused on science content, inquiry-based instruction, technological applications, and differentiated instruction within problem-based curriculum units. Statistically significant gains in science process skills, science concepts, and science content knowledge were found among gifted students in the treatment group when compared with gifted students in the comparison group.