  The process e+e- --> W+W- provides a valuable laboratory to test the Standard
Model (SM) and to search for new physics. The most general helicity amplitudes
for this process require the introduction of nine form-factors which we
calculate in the context of SU(2) X U(1) gauge-invariant extensions of the SM.
The contributions of new physics are parametrized via an effective Lagrangian
constructed from the light fields. Because the mechanism of electroweak
symmetry-breaking remains an open problem we consider both the effective
Lagrangian with a linearly realized Higgs sector, i.e. with a light physical
Higgs boson, and the effective Lagrangian which utilizes a nonlinear
realization of the Higgs mechanism. The use of an effective Lagrangian allows
one to calculate consistently nonstandard contributions to e+e- --> W+W-
amplitudes as well as the nonstandard contributions to other processes. We
study the interplay of the low-energy and Z-pole measurements with measurements
via the processes e+e- --> fermion pairs and e+e- --> W+W- at LEP II or a
future linear e+e- collider. Concrete relationships between operators of the
linear and nonlinear realizations are presented where possible.
