Sherin and van Es are the key scholars who popularised the ‘teacher noticing’ construct (Sherin, 2001; van Es & Sherin, 2002). Two premises underpinned their research on teacher noticing. First, teachers, like members of other professions, develop a unique way of seeing relevant to their profession (i.e., professional vision (Goodwin, 1994)). Second, effective teachers are able to identify salient classroom interactions within the visually complex classroom environment. In the context of reform, when teachers are required to teach in student-centred and responsive ways, noticing and interpreting classroom events become critical skills. Based on theories of human cognition and their own empirical research, Sherin and van Es conceptualised teacher noticing as comprising two dynamically interacting processes: selective attention and knowledge-based reasoning (Sherin, 2007, 2017). Selective attention refers to how teachers attend to some interactions while filtering out others, whereas knowledge-based reasoning involves teachers drawing on their knowledge and experience to interpret what they notice (Sherin, 2007, 2017).

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It is worth pointing out that teacher noticing is not ‘simply another category of teacher knowledge’ but involves how teachers ‘at a fine-grained level, interact with the classroom world’ (Sherin, et al., 2001, p. 5). The teacher noticing construct underlies the in-the-moment perceptual and cognitive thinking processes that occur during the interactive phase of teaching when teachers are ‘confronted with a blooming, buzzing confusion of sensory data’ (Sherin, et al., 2001, p. 4). These perceptual and cognitive thinking processes are critical for bridging teachers’ thoughts and actions as well as mediating teachers’ knowledge and practices.

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Since its inception, the teacher noticing construct has been understood and used in different ways. See Chan et al. (2020) for more details. Lam and Chan (2020) distinguished two types of noticing - in-the-moment noticing and delayed noticing - used in existing studies on science teachers’ noticing. Delayed noticing describes teachers’ perceptual and cognitive thinking processes occurring when teachers reflect on their teaching practices captured in video records. In this context, teachers are removed from the immediacy of teaching and the demands of decision-making under in-the-moment pressures. Teachers are often given a chance to review and revisit evidence of student thinking to make sense of and decide how to respond to it. This contrasts with in-the-moment noticing, in which teachers face the pressure of reacting in the moment. In such a context, they must home in on evidence of student thinking, which may be transient, given the complexity of classroom interactions; draw inferences regarding that evidence; and make on-site teaching decisions in seconds.