  We present a semiclassical non-perturbative approach for calculating the
preheating process at the end of inflation. Our method involves integrating out
the decayed particles within the path integral framework and subsequently
determining world-line instanton solutions in the effective action. This
enables us to obtain the effective action of the inflaton, with its imaginary
part linked to the phenomenon of particle creation driven by coherent inflaton
field oscillations. Additionally, we utilize the Bogoliubov transformation to
investigate the evolution of particle density within the medium after multiple
inflaton oscillations. We apply our approach to various final state particles,
including scalar fields, tachyonic fields, and gauge fields. The
non-perturbative approach provides analytical results for preheating that are
in accord with previous methods.
