  We show that the non-linear evolution of long wavelength perturbations may be
important in a wide class of inflationary scenarios. We develop a solution for
the evolution of such nonlinear perturbations which is exact to first order in
a gradient expansion. As a first application, we demonstrate that in single
field models of inflation there can be no parametric amplification of
super-Hubble modes during reheating. We consider the implications of the
solution for recent discussions of the back-reaction effect of long wavelength
perturbations on the background geometry, give a new derivation of the equation
of motion of stochastic inflation, and demonstrate that if the (generalized)
slow-rolling condition is not satisfied, then inevitably long wavelength vector
modes for gravitational fluctuations will be generated.
