  In inflationary cosmology, the particles constituting the Universe are
created after inflation in the process of reheating due to the interaction with
the oscillating inflaton field. We briefly review the basics of the slow
reheating, and the stage of fast preheating, when the particles are created
explosively in the regime of parametric resonance. The non-perturbative,
out-of-equilibrium character of the parametric resonance changes many features
of reheating. For these proceedings, we will highlight a few aspects of
preheating: the structural dependence of the parametric resonance on the
inflationary model, including $V(\phi)={m^2 \over 2}\phi^2$, ${\lambda\over
4}\phi^4$, $1-\cos {\phi \over f}$; ``rescattering'' of created particles; and
phase transitions after inflation.
