Whole Number Arithmetic (WNA) appears as the very first topic in school mathematics and establishes the foundation for later mathematical content. Without solid mastery of WNA, students may experience difficulties in learning fractions, ratio and proportion, and algebra. The challenge of students’ learning and mastery of fractions, decimals, ratio and proportion, and algebra is well documented. Most of this research has focused on either fractions, decimals, ratio and proportion, algebra, or WNA. There is a lack of research that addresses the connection between these relevant topics. Within WNA, most research focuses on counting, computation, or solving word problems. There is a lack of research that investigates connections within WNA. This special issue is intended to bridge this research gap by explicitly highlighting the conceptual knowledge of counting, calculations, and quantity relationships, as well as the structure of word problems within and beyond WNA.