This case study describes the interactions between pedagogical content knowledge (PCK) and practitioner inquiry as observed in a residency-model teacher preparation program. In the Math and Science Teacher Education Residency (MASTER), novice teachers engaged in iterative cycles of assessing student thinking and using that assessment to reflect on and inform instructional decisions. Through an analysis of resident presentations on their inquiry and examination of student work, this case study explores the synergy between inquiry processes and PCK development and demonstrates that using teacher inquiry as a vehicle for building and refining PCK is a promising practice. MASTER pre-service teachers utilized canonical PCK to analyze assessments, develop goals for student learning, plan for instruction, and then interpret the results of further assessment to understand the impact of their instructional strategies and how they might revise their strategies for future lessons. Implementing the inquiry process provided opportunities for residents to build their personal PCK through their reflection on and analysis of their students' learning and their own teaching. Collaboration protocols allowed coaches and peers to provide prompts that engaged directly with the content the resident was teaching, pushed the resident to deepen thinking, and developed shared understandings through integrating PCK from multiple individuals. The case study investigates the ways in which PCK is foundational to conducting teacher inquiry but is also built and refined by testing through the "inaction" reflection and revision fostered by the teacher inquiry process. [For the complete volume, "Pedagogical Content Knowledge in STEM: Research to Practice. Advances in STEM Education," see ED613708.]