In this article, the author describes a project in which Collaborative Inquiry, or CI (Bray et al. 2000), was used by a small inquiry group consisting of a high school science teacher and two university educators, Marie Cashion and the author herself, Karen Goodnough, to explore Problem-Based Learning (PBL). In CI groups, co-learners work through repeated cycles of planning, reflection, and action as they explore an issue or question of importance to the group. This two-year ongoing collaborative project provides insight into how effective professional development projects can be structured to enhance teachers' knowledge and classroom practice and to contribute to educational theory. The study was guided by several research questions: (1) What challenges and issues will schools encounter as they engage in PBL curriculum development and implementation?; (2) How will the teacher's professional knowledge (beliefs, values, and classroom practice) change as a result of this experience?; (3) How will students perceive PBL, a new approach to learning?; and (4) How will CI foster teacher learning and contribute to an understanding of the nature of PBL?