The Gerasimov-Drell-Hearn sum rule

This contribution covers the status of theory concerning the GDH sum rule as well as the experimental approaches and their results for the absorption of real and virtual photons. We point out that the so-called No-Subtraction hypothesis, often considered the weakest part of the derivation of the GDH sum rule, in fact follows from unitarity and does not impair the fundamental character of the GDH sum rule. The experimental data verify the GDH sum rule for the proton at the level of 8% including the systematic uncertainties from extrapolations to unmeasured energy regions. For the GDH sum rule on the neutron and the isovector case we find unexpected contributions at photon energies above 1 GeV.