Up until now, teacher’s formative assessment practices have been apprehended in a normative and prescriptive perspective. This research approaches the question by a different angle by looking at the practical know-how of elementary school teacher’s on formative assessment inspired by Schön’s work (1983) on knowledge building in every day practical experiences. It involves five elementary school teachers who participated in a series of three individual interviews each, pertaining to videos filmed in their respective classrooms. The co-analysis of these filmed episodes of formative assessment helped to shed light on an ensemble of “ways of doing” formative assessment and their analysis allowed us to distinguish strategic know-hows, know-hows pertaining to the work process, some pertaining to conditions of practice, to the pedagogical relation, to the know-hows relating to the teacher’s posture and theoretical know-hows.