In this chapter, we will examine mental calculation with rational numbers, namely fractions, decimals and percentages (called part-whole numbers in the literature). First, we will refer to rational numbers to illustrate some concepts and to determine difficulties children encounter during their transition from natural to rational numbers. Then we present the different strategies that students use for rational numbers. We will present children’s behaviours with regard to these kinds of calculations based on current research literature and findings of local research. We illustrate results using studies in international literature. We will also present results on the comparison of fractions. Finally, we will display research findings from teachers and studies on instruction in rational numbers.