This exploratory study of elementary school science examines questions central to policy, practice and research on formative assessment: What is the quality of teachers' content-pedagogical and assessment knowledge? What is the relationship between teacher knowledge and assessment practice? What is the relationship between teacher knowledge, assessment practice and student learning? Drawing on multiple measures, hierarchical linear modelling and path analysis, results suggest that despite weaknesses in teachers' content-pedagogical and assessment knowledge, teachers' formative assessment practices are positively related to student learning. Relationships between teachers' knowledge and assessment practices are mixed. Findings underscore both the potential and challenge of bringing effective formative practice to fruition as well as the need for continued research.