  We report on a study of the physics potential of linear $e^+e^-$ colliders.
Although a linear collider (LC) would support a broad physics program, we focus
on the contributions that could help elucidate the origin of electroweak
symmetry breaking. Many extensions of the standard model have a decoupling
limit, with a Higgs boson similar to the standard one and other, higher-mass
states. Mindful of such possibilities, we survey the physics of a (nearly)
standard Higgs boson, as a function of its mass. We also review how
measurements from an LC could help verify several well-motivated extensions of
the standard model. For supersymmetry, we compare the strengths of an LC with
the LHC. Also, assuming the lightest superpartner explains the missing dark
matter in the universe, we examine other places to search for a signal of
supersymmetry. We compare the signatures of several scenarios with extra
spatial dimensions. We also explore the possibility that the Higgs is a
composite, concentrating on models that (unlike technicolor) have a Higgs boson
with mass of a few hundred GeV or less. Where appropriate, we mention the
importance of high luminosity, for example to measure branching ratios of the
Higgs, and the importance of multi-TeV energies, for example to explore the
full spectrum of superpartners.
