Formative assessment, assessment used to inform subsequent learning, can have a powerful positive impact on student achievement, but little empirical work has been conducted to investigate the role of teachers' knowledge in its practice. This study investigated reciprocal relations between elementary science teachers' formative assessment practices and their pedagogical content knowledge (PCK). Data sources included video records and artifacts from a professional development (PD) program that engaged 11 teachers in collaborative analysis of their students' work. Analysis of the data showed that teachers' PCK was a resource in all aspects of their formative assessment practice, and teachers most frequently made use of their existing knowledge of instructional strategies and curriculum. It also showed that teachers constructed PCK through formative assessment by building and refining knowledge of curricular goals and student understanding over multiple PD sessions. Findings suggest that formative assessment can be a powerful opportunity for teachers to use, integrate, and generate PCK but that teachers require additional resources to build knowledge of instructional strategies. (Contains 2 tables, 1 footnote and 2 figures.)