This article introduces this special issue by addressing the types of assessment practices that are possible within STEM inquiry classrooms, where the focus is on noticing, developing, and improving inquiry competencies. Three domains are explored: informal formative assessment occurring in classroom interactions; structured formative assessment when teachers or peers collect information at a planned time; ongoing evolution of teachers’ knowledge and practices. We analyse how classroom formative assessment practices develop alongside and support inquiry learning. We report on how STEM teachers might be supported in transforming their teaching practices and assessment methods in tandem. We focus on the role of classroom discussion to challenge students’ ideas and make explicit their reasoning. We highlight how interactions between teachers and students support students in being more active in the assessment process. Combining inquiry learning with adapted assessment practices requires teachers to make significant changes to their practices. They need to be aware of how students develop their learning. Such professional moves are complex because implementing formative and summative assessments in a complementary manner remains a challenge for teachers and researchers. We emphasise how the collaboration between researchers and teachers support teachers in making changes to their professional knowledge, beliefs, and classroom practices.