Nadharia ya namba (kwa Kiingereza Theory of numbers; awali: arithmetic) ni tawi la hisabati linalochunguza hasa nambakamili. Mwanahisabati Mjerumani Carl Friedrich Gauss (1777–1855) alisema, "Hisabati ndiyo malkia wa sayansi zote, na nadharia ya namba ndiyo malkia wa hisabati zote." Already in 1921, T. L. Heath had to explain: "By arithmetic, Plato meant, not arithmetic in our sense, but the science which considers numbers in themselves, in other words, what we mean by the Theory of Numbers." (Heath 1921, p. 13) Take, for example, Serre 1973. In 1952, Davenport still had to specify that he meant The Higher Arithmetic. Hardy and Wright wrote in the introduction to An Introduction to the Theory of Numbers (1938): "We proposed at one time to change [the title] to An introduction to arithmetic, a more novel and in some ways a more appropriate title; but it was pointed out that this might lead to misunderstandings about the content of the book." Apostol, Tom M. (1976).