In this study, five elementary teachers and a university researcher developed and implemented problem-based learning (PBL) experiences in the context of science teaching and learning. Collaborative inquiry was adopted as a methodology, while a variety of qualitative methods were used to examine the engagement and development of teachers' pedagogical content knowledge (PCK). A PCK model is used as a framework to examine teachers' professional knowledge growth in areas such as orientations to teaching science, knowledge of science curriculum, knowledge of students' understanding of science, knowledge of assessment, and knowledge of instructional strategies. Implications for how teachers may be supported when adopting instructional innovations are discussed.