  Reheating is a process where the energy density of a dominant component of
the universe other than radiation, such as a matter component, is transferred
into radiation. It is usually assumed that the temperature of the universe
decreases due to cosmic expansion even during the reheating process, in which
case the maximal temperature of the universe is much higher than the reheat
temperature. We point out that the temperature of the universe during reheating
may in fact increase in well-motivated scenarios. We derive the necessary
conditions for the temperature to increase during reheating and discuss
concrete examples involving a scalar field. We comment on implications for
particle physics and cosmology due to an increasing temperature during
reheating.
