In this paper, the authors describe a model of professional development for early childhood science teachers based on their experiences over the past two years with Project ASTER: Active Science Teaching Encourages Reform. In this project, they have identified seven elements that constitute a quality professional development program. These elements include: (1) strong partnership between school districts and institutions of higher education; (2) unique collaboration between science educators and scientists; (3) three-phase training program designed around the school district's adopted course of study and national science standards; (4) integration of community resources; (5) partnership with preservice teachers; (6) development of teacher leaders; and (7) comprehensive program evaluation. The model of professional development implemented in this project is based upon the recommendations from Haney and Lumpe (1995) and Loucks-Horsley, Hewson, Love, and Stiles (1998) that mirror suggestions found in various national curriculum science standards. Moreover, their model emphasized enabling the teachers to become engaged with strategies that consist of ongoing processes rather than "one-shot" workshops. In addition, this study sought to explore the impact of the ASTER model of professional development on teacher self-efficacy and perceptions about science teaching. (Contains 2 tables.)