在数学中，代数数论是数论的一支，其中我们将「数」的概念延伸，以解决具体的数论问题。我们在代数数论中考虑代数数，这类数是有理系数多项式的根。与此相关的概念是数域，这是有理数域的有限扩张。在此框架下能推广整数为代数整数，并研究一个数域里的代数整数。

代数整数在加法、减法与乘法下构成一个环，但整数的许多性质并不能推广到一般数域里的代数整数上，其中一个例子是素因数分解的唯一性（又称算术基本定理），这是十九世纪数学家试图证明费马大定理时遇到的主要阻碍，然而代数数论的应用不仅止于此。数学中一些较深入的理论有助于让我们了解代数数与代数整数的性质——包括伽罗瓦理论、伽罗瓦上同调、类域论、表示理论与L-函数的相关理论等等。

Algebraic number theory is a major branch of number theory that studies algebraic structures related to algebraic integers. This is generally accomplished by considering a ring of algebraic integers O in an algebraic number field K/Q, and studying their algebraic properties such as factorization, the behaviour of ideals, and field extensions. In this setting, the familiar features of the integers—such as unique factorization—need not hold. The virtue of the primary machinery employed—Galois theory, group cohomology, group representations, and L-functions—is that it allows one to deal with new phenomena and yet partially recover the behaviour of the usual integers.