  The reheating stage in post-inflationary cosmologies is reanalyzed. New
techniques from non-equilibrium quantum field theory allow a consistent
derivation of the equation of motion including the non-linearity of the
dynamics. These offer a rationale for the elementary theory of reheating based
on single particle decay which is seen to be valid only in the linear regime of
coherent oscillations of the scalar field. A new non-perturbative mechanism of
induced amplification of quantum fluctuations is introduced and studied in
detail, both analytically and numerically. This is a non-linear mechanism that
is typically a far more efficient way of transfering energy out of the zero
mode and into production of lighter particles than single particle decay.
Thermalization is discussed and we estimate the reheating temperature to be of
the order of the inflaton mass, thus providing a potential solution to the
Polonyi and moduli problems.
