  In the confinement regime at Q^2=0 the Gerasimov-Drell-Hearn (GDH) Sum Rule
connects static properties of the nucleon with the difference of spin dependent
doubly polarized total absorption cross sections of real photons. Now, for the
first time this fundamental sum rule is verified by the GDH-Collaboration with
circularly polarized real photons and longitudinally polarized nucleons at the
two accelerators ELSA and MAMI. The integral (sum) of the GDH Sum Rule can be
generalized to the case of virtual photons. This allows to establish a Q^2
dependency and to study the transition to the perturbative regime of QCD.
Ultimately, the GDH Sum Rule can be related to the Bjorken and the Ellis-Jaffe
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 shown 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.
