This volume contains the proceedings of the twenty-third study led by the International Commission on Mathematical Instruction (ICMI). The study addresses, for the first time, mathematics teaching and learning in primary school (and pre-school as well) for all, taking into account international perspectives including socio-cultural diversity and institutional constraints. Whole number is the core content area, which is regarded as foundational for later mathematics learning; its teaching and learning are thus very important due to larger impact for later mathematics knowing. The study was lauched by ICMI at the end of 2012, with the appointment of two co-chairs (Maria G. (Mariolina) Bartolini Bussi and Xuhua Sun) and of the International Program Committee (IPC), which on behalf of ICMI is responsible for conducting the Study: Berinderjeet Kaur, Hamsa Venkat, Jarmila Novotna, Joanne Mulligan, Lieven Verschaffel, Maitree Inprasitha, Sybilla Beckmann, Sarah González; Abraham Arcavi (ICMI Secretary General), Ferdinando Arzarello (ICMI President), Roger E. Howe (ICMI liason). During 2013 an intense mail exchange was realised within the IPC, in order to share the rationale, the goals and the steps of the forthcoming ICMI Study.