We report on the calculation of the NLO QCD corrections to vector boson pair production
in gluon fusion at hadron colliders. We focus in particular on the on-shell production of vector bosons, which is of fundamental importance, among the others, to check the consistency of the electroweak sector of the Standard Model. For similar studies, including the interference of the off-shell prompt $gg \to V_1V_2$ amplitudes with the Higgs production amplitudes $gg \to H \to V_1 V_2$, we refer to the proceedings of this conference.