Amount of Substance and the mole

The use of the term amount of substance of an elementary entity is expounded. Methods of measurement of the ratio of two amounts of substance are described. The SI unit of amount of substance, the mole, is introduced. Quantities involving amount of substance, including molar mass, molar quantities in general, the Avogadro constant, the molar gas constant, and the Faraday constant, are defined. An historical account is given of the notion of amount of substance and of the unit mole. Some personal views are advanced about the desirability of a new name for amount of substance and for derived quantities.