  In this report I surveyed physics potential of the gamma-gamma option of a
Linear e+e- Collider with the following questions in mind: What new discovery
can be expected at a gamma-gamma collider in addition to what will be learned
at its `parent' e+e- Linear Collider? By taking account of the hard energy
spectrum and polarization of colliding photons, produced by Compton
back-scattering of laser light off incoming e- beams, we find that a
gamma-gamma collider is most powerful when new physics appears in the neutral
spin-zero channel at an invariant mass below about 80% of the c.m. energy of
the colliding e-e- system. If a light Higgs boson exists, its properties can be
studied in detail, and if its heavier partners or a heavy Higgs boson exists in
the above mass range, they may be discovered at a gamma-gamma collider. CP
property of the scalar sector can be explored in detail by making use of linear
polarization of the colliding photons, decay angular correlations of final
state particles, and the pattern of interference with the Standard model
amplitudes. A few comments are given for SUSY particle studies at a gamma-gamma
collider, where a pair of charged spinless particles is produced in the s-wave
near the threshold. Squark-onium may be discovered. An e^\pm gamma collision
mode may measure the Higgs-Z-gamma coupling accurately and probe flavor
oscillations in the slepton sector. As a general remark, all the Standard Model
background simulation tools should be prepared in the helicity amplitude level,
so that simulation can be performed for an arbitrary set of Stokes parameters
of the incoming photon beams.
