Engaging primary and elementary students in meaningful, relevant science learning is challenging. PBL is an instructional approach that provides a means to foster meaningful science learning while enhancing teachers' pedagogical content knowledge (PCK). This paper reports on the experiences of a teacher inquiry group consisting of five teachers (K-5) and a university researcher as they adopted a nine-step problem design model to develop PBL experiences. The objectives of the study were to examine how various facets of teachers' pedagogical content knowledge are engaged as they design PBL modules; describe how teachers engage with a nine-step problem design model; and document teachers' perceptions of the strengths and limitations of the nine-step problem design model.