The physical quantity “amount of substance” is measured in the unit “mole” (symbol: mol). In and because of its current definition (via 12 g of ¹²C) it is closely related to another SI base unit, namely the mass unit kilogram. In principle there is an important difference between mass and amount of substance, because in case of the amount of substance not only the amount of material is relevant, but also the unambiguous specification of the entities forming this amount of material. The practical realization and dissemination of the unit mole is usually based on materials characterized with regard to their purity. The amount of substance is then derived from the mass, the molar mass, and said purity. In the wake of the redefinition of “The International System of Units” (SI) aiming at a set of defining fundamental constants, also the definition of the “mole” will be rewritten. In its new definition it will be based on a fundamental constant, namely the Avogadro constant. This way also the link to the definition of the kilogram will be broken, underpinning the importance of the “mole” as a base unit in its own right.