  The environment in which bremsstrahlung and pair creation occurs can strongly
affect cross sections for these processes. Because ultra-relativistic
electromagnetic interactions involve very small longitudinal momentum
transfers, the reactions occur gradually, spread over long distances. During
this time, even relatively weak factors can accumulate enough to disrupt the
interaction.
  This review will discuss a variety of factors which can suppress
bremsstrahlung and pair production, as well as related effects involving
beamstrahlung and QCD processes. After surveying different theoretical
approaches, experimental measurements from the early cosmic ray studies to the
recent detailed measurements by the SLAC E-146 collaboration will be discussed.
