Over the past three years, the authors' team at Education Development Center, Inc. (EDC) has been researching a professional development program in science, "Foundations of Science Literacy" ("FSL"), for preschool lead and assistant Head Start teachers in Massachusetts and Rhode Island. Year 1 was a pilot year, so it is the data from Year 2 on which the authors' report in this paper. "Foundations of Science Literacy" is designed to respond to the urgent call to prepare preschoolers for tomorrow and respond to Hawkins's eloquent plea for equity. The authors' research is designed to answer two important questions germane to this paper: (1) Does "FSL" impact Head Start teachers' practices in inquiry-based science instruction for four-year-old children?; and (2) Does "FSL" impact Head Start children's early science knowledge and skills? The objective of the presentation will be to report the principal teacher-, classroom-, and child-level findings from the Year 2 implementation, and to discuss these findings in the context of what makes for a successful professional development program in early science. The research took place in the metro-west and southern sections of Massachusetts. The results indicate that "FSL" had a strong impact on teachers' science knowledge and classroom practices in inquiry-based science instruction, and that children in "FSL" classrooms showed a trend towards greater improvement in their understanding of basic physical science principles and their use of science inquiry skills. In addition, the findings have lead to specific revisions in "FSL", including the use of more constrained classroom assignments, allowing for a better alignment between "FSL" and the "Preschool Assessment of Science" ("PAS") content, and a stronger emphasis on the growth of children's reflective capacity in the context of "FSL". The authors conclude that successful development of professional development programs requires evaluation at every level in the "causal chain" that connects teaching and learning--from teacher's content knowledge, to their ability to apply it in their classrooms, to children's ability to engage in focused science activities with genuine conceptual content. (Contains 4 tables and 1 figure.)