At very high energies, the bremsstrahlung and pair production cross sections
exhibit complex behavior due to the material in which the interactions occur.
The cross sections in dense media can be dramatically different than for
isolated atoms. This writeup discusses these in-medium effects, emphasizing how
the cross section has different energy and target density dependencies in
different regimes. Data from SLAC experiment E-146 will be presented to confirm
the energy and density scaling. Finally, QCD analogs of the electrodynamics
effects will be discussed.Comment: 10 pages with 7 figures. Invited talk presented at the Workshop on
Electromagnetic Probes of Fundamental Physics, Oct. 16-21, 2001, Erice, Ital
