  Due to coherence, there are strong electromagnetic fields of short duration
in very peripheral collisions. They give rise to photon-photon and
photon-nucleus collisions with high flux up to an invariant mass region
hitherto unexplored experimentally. After a general survey photon-photon
luminosities in relativistic heavy ion collisions are discussed. Special care
is taken to include the effects of strong interactions and nuclear size. Then
photon-photon physics at various invariant mass scales is discussed. Invariant
masses of up to about 100 GeV can be reached at LHC, and in addition the
potential for new physics is available. Photonuclear reactions and other
important background effects, especially diffractive processes are also
discussed. Lepton-pair production, especially electron-positron pair production
is copious. Due to the strong fields there will be new phenomena, especially
multiple e+e- pair production.
