  We review high--energy scattering processes that are sensitive to the
hadronic structure of the photon, describing theoretical predictions as well as
recent experimental results. These processes include deep--inelastic
electron--photon scattering at \eplem\ colliders; and the production of jets,
heavy quarks and isolated photons in the collision of real photons at \eplem\
colliders, as well as in photon--proton collisions at $ep$ colliders. We also
comment on ``minijet'' based calculations of total $\gamma p$ and
  \gamgam\ cross--sections, and discuss the possibility that future linear
  \eplem\ colliders might produce very large photon fluxes due to the
  ``beamstrahlung'' phenomenon; in the most extreme cases, we predict more than
one hadronic \gamgam\ event to occur at every bunch crossing.
