Noticing is a momentary act. Enriching the possibility of noticing something specific in the future marks the beginning of an action labelled "professional development". Thus, what happens as a result of noticing is what matters for the teacher, but what really matters is what happens for the learner. Whether something noticed enables a passing remark, acts an object of reflection, stimulates a shift of attention, provides an action for pre-flection, or instantiates some theoretical construct, the consequence of noticing is much more complex than simply noticing, involving as it does the power of attention, and implicitly all other aspects of the human psyche. As a discipline, noticing encompasses all of these aspects. As a domain of research, noticing is sometimes divided into component parts, with distinctions between different kinds of things worth noticing and distinctions between different ways of behaving as a consequence of noticing. As a subject of research, the key issues about noticing are how it can be promoted, fostered, provoked and even directed, and what it enables. This paper re-integrates various disparate aspects of noticing appearing in the literature, in order to stress the complexity of sensitising oneself to notice, and informing one's future practice through noticing opportunities to act freshly. The reports in this issue are used as an indication of the range of things which people are encouraged to notice (be sensitised to notice) as part of their professional development and practice in mathematics education. This occasions reflection on the complexity of researching, not only what people notice and attend to, but how their noticing might be influenced as part of their professional development, and more importantly, what consequences of such noticing might be anticipated, such as how it might influence future practice.