This paper provides a critical analysis of a professional development model that proposes to improve inquiry science instruction by addressing two components of teacher knowledge: pedagogy and science content. Although this represents an improvement over professional development models that focus on a single area of teacher knowledge, this paper suggests that, in order to impact inquiry science instruction, three pieces of teacher knowledge are required: pedagogy, science content, and pedagogical content knowledge. Without all three components, the impact on classroom instruction will be limited. (Contains 17 references.) (Author/MVL)