ABSTRACT
Starting in 2011, a separate Thematic Working Group (TWG) has been devoted to research on teaching and learning arithmetic and number systems at Congress of European Research in Mathematics Education (CERME). This chapter explains the different notions of number sense that are used at CERME and complement these by definitions from other perspectives, especially from cognitive- and neuro-psychology. It addresses three main questions: what are the different understandings of number sense within CERME and how do they relate to one another? What is suggested in order to foster the development of number sense? and What is the scope of number sense in terms of number domains? In line with the didactical understanding of number sense, some papers make suggestions how the development of number sense might be fostered. The synopsis of research referring to number sense in CERME shows that number sense is a multi-faceted concept.
