When thinking about mathematical abilities, mental arithmetic is among the most prominent skills that come to mind. We all spend years in school to acquire the most basic mathematical competencies for solving simple addition, subtraction, multiplication, and division problems. Later in school, we move further and further away from manipulating concrete numerical quantities to learning more abstract mathematical skills such as solving equations with one or more unknowns (2x + 3y = 23). Of course, this is only where “real” mathematics starts. In this chapter, I will mainly focus on the basic mathematical concepts of arithmetic for two reasons. First, these are the essential skills every child in Western societies needs to acquire and hence we all can refer to that. Second, most psychological and neurocognitive research has focused on these basic skills since they represent the foundations for any skills acquired later on.