The purpose of this study was to examine the impact of professional development on teachers' knowledge of teaching science and sense of efficacy regarding the teaching of science. In addition, the study explores the association between knowledge of teaching science and efficacy regarding the teaching of science. Participants included 28 early childhood teachers from seven school districts, predominantly rural with high incidence of poverty. Results indicate gains in teachers' knowledge of teaching science and their sense of efficacy after participating in professional development. Implications include the need to integrate STEM (science, technology, engineering, and mathematics) content and language arts curricula into early childhood classrooms.