  High intensity back-scattered laser beams will allow the efficient conversion
of a substantial fraction of the incident lepton energy into high energy
photons, thus significantly extending the physics capabilities of an
electron-electron or electron-positron linear collider. The annihilation of two
photons produces C=+ final states in virtually all angular momentum states. The
annihilation of polarized photons into the Higgs boson determines its
fundamental two-photon coupling as well as determining its parity. Other novel
two-photon processes include the two-photon production of charged lepton pairs,
vector boson pairs, as well as supersymmetric squark and slepton pairs and
Higgstrahlung. The one-loop box diagram leads to the production of pairs of
neutral particles. High energy photon-photon collisions can also provide a
remarkably background-free laboratory for studying possibly anomalous $W W$
collisions and annihilation. In the case of QCD, each photon can materialize as
a quark anti-quark pair which interact via multiple gluon exchange. The
diffractive channels in photon-photon collisions allow a novel look at the QCD
pomeron and odderon. Odderon exchange can be identified by looking at the heavy
quark asymmetry. In the case of electron-photon collisions, one can measure the
photon structure functions and its various components. Exclusive hadron
production processes in photon-photon collisions test QCD at the amplitude
level and measure the hadron distribution amplitudes which control exclusive
semi-leptonic and two-body hadronic B-decays.
