In mathematics, **ideal theory** is the theory of ideals in commutative rings; and is the precursor name for the contemporary subject of commutative algebra. The name grew out of the central considerations, such as the Laskerâ€“Noether theorem in algebraic geometry, and the ideal class group in algebraic number theory, of the commutative algebra of the first quarter of the twentieth century. It was used in the influential van der Waerden text on abstract algebra from around 1930.

The ideal theory in question had been based on elimination theory, but in line with David Hilbert's taste moved away from algorithmic methods. GrÃ¶bner basis theory has now reversed the trend, for computer algebra.

The importance of the ideal in general of a module, more general than an *ideal*, probably led to the perception that *ideal theory* was too narrow a description. Valuation theory, too, was an important technical extension, and was used by Helmut Hasse and Oscar Zariski. Bourbaki used *commutative algebra*; sometimes *local algebra* is applied to the theory of local rings. D. G. Northcott's 1953 Cambridge Tract *Ideal Theory* (reissued 2004 under the same title) was one of the final appearances of the name.

In political philosophy, **ideal theory** refers to argument concerning political or social arrangements under favorable assumptions. The phrase is associated with the work of John Rawls.