The Herbrand-Ribet theorem is a strengthening of Kummer's theorem to the effect that the prime p divides the class number of the cyclotomic field of p-th roots of unity if and only if p divides the denominator of the nth Bernoulli number B_n for some n, 0 < n < p − 1. The Herbrand-Ribet theorem specifies what, in particular, it means when p divides such an B_n.

The Galois group Σ of the cyclotomic field of pth roots of unity for an odd prime p, with ζ^p = 1, consists of the p − 1 group elements σ_a, where σ_a is defined by the fact that σ_a(ζ) = ζ^a. As a consequence of the little Fermat theorem, in the ring of p-adic integers we have p − 1 roots of unity, each of which is congruent mod p to some number in the range 1 to p − 1; we can therefore define a Dirichlet character ω (the Teichmüller character) with values in by requiring that for n relatively prime to p, ω(n) be congruent to n modulo p. The p part of the class group is a -module, and we can apply elements in the group ring to it and obtain elements of the class group. We now may define an idempotent element of the group ring for each n from 1 to p − 1, as

We now can break up the p part of the ideal class group G of by means of the idempotents; if G is the ideal class group, then G_n = ε_n(G).

Then we have the theorem of Herbrand-Ribet: G_n contains no elements if and only if p divides the Bernoulli number B_p−n. The part saying p divides B_p−n if G_n is not trivial is due to Herbrand. The converse, that if p divides B_{p−n then Gn is not trivial is due to Ribet, and is considerably more difficult. By class field theory, this can only be true if there is an unramified extension of the field of pth roots of unity by a cyclic extension of degree p which behaves in the specified way under the action of Σ; Ribet proves this by actually constructing such an extension.}