  The Drell-Hearn-Gerasimov (DHG) sum rule relates the helicity structure of
the photoabsorption cross section to the anomalous magnetic moment of the
nucleon. It is based on Lorentz and gauge invariance, crossing symmetry,
causality and unitarity. A generalized DHG sum rule my be derived for virtual
photons. At low momentum transfer this generalized sum rule is saturated by the
resonance region, at high momentum transfer it may be expressed by the parton
spin distributions measured in deep inelastic scattering. The
longitudinal-transverse interference determines the Cottingham sum rule, which
is related to the electric and magnetic form factors over the whole range of
momentum transfer.
