What Is Mathematics? is a classic book by Richard Courant and Herbert Robbins, published by Oxford University Press. Written in the belief that "the traditional place of mathematics in education is in grave danger," it is an introduction to mathematics, intended to offer "vantage points from which the substance and driving forces of modern mathematics can be surveyed" both by students and by the general public.

First published in 1941, it discusses number theory, geometry, topology, and calculus. A posthumous edition was published in 1996 with an additional chapter on recent progress in mathematics, including the proofs of the four-color map theorem and Fermat's last theorem, written by Ian Stewart.

The book was based on Courant's course material. Although Robbins assisted in writing a large part of the book, he had to fight for authorship. Courant alone held the copyright for the book. This resulted in Robbins receiving a smaller share of the royalties.

According to Constance Reid, Courant finalized the choice of the title after discussing it with Thomas Mann.

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