Thematic Working Group 2 focused on six questions related to teaching and learning of arithmetic and number systems from grades 1 to 12: 1) What is the interplay between conceptual understanding and procedural skills for number operations, and how should these two aspects be balanced in the design of learning environments? 2) What does it mean to operate flexibly with numbers, and what knowledge and skills are required therefore? 3) What are the roles of models in teaching and learning of arithmetic and number systems, and how do they support flexibility and conceptual understanding? 4) What are effective approaches for teaching and learning of arithmetic and number systems in inclusive education? How can this content area be taught for students with special needs? 5) What aspects of number theory (including specific reasoning) are supposed to be taught in primary and secondary school, and how can this be done? 6) How is it possible to support and analyze long-term learning processes from grade 1 to grade 12? How can different transitions that come with a long-term learning process be taken into account, especially the primary-secondary transition, but also the secondary-tertiary transition.