  The collisions of high energy photons produced at an electron-positron
collider provide a comprehensive laboratory for testing QCD, electroweak
interactions, and extensions of the Standard Model. The luminosity and energy
of the colliding photons produced by back-scattering laser beams is expected to
be comparable to that of the primary $e^+e^-$ collisions. In this overview, we
shall focus on tests of electroweak theory in photon-photon annihilation,
particularly $\gamma\gamma \rightarrow W^+ W^-$, $\gamma\gamma \rightarrow $
Higgs bosons, and higher-order loop processes, such as $\gamma\gamma
\rightarrow \gamma\gamma, Z \gamma$ and $ZZ.$ Since each photon can be resolved
into a $W^+ W^-$ pair, high energy photon-photon collisions can also provide a
remarkably background-free laboratory for studying $WW$ collisions and
annihilation. We also review high energy $\gamma\gamma$ tests of quantum
chromodynamics, such as the scaling of the photon structure function, $t \bar
t$ production, mini-jet processes, and diffractive reactions.
