  In diffractive interactions of protons or nuclei a violent collision can
occur that leaves the forward going particle completely intact -with
probability determined by the structure of the proton or nucleus.
  At very high energies these collisions also occur with both incident
particles remaining intact. This is called central exclusive production. If a
new particle, such as the Higgs boson, were produced exclusively this process
would give a precise measurement of its mass and test for expected properties
of the Higgs. Because of its unusual features this process is also a promising
discovery tool.
  In this paper I focus on analogous electromagnetic processes because many
aspects apply to both- particularly the role of coherence. Also, topics in
diffraction with nuclear beams are based on electromagnetic interactions. I
also discuss two proposed measurements in ATLAS with Pb beams and with proton
beams (diffractive Higgs production).
