  Our understanding of the state of the universe between the end of inflation
and big bang nucleosynthesis (BBN) is incomplete. The dynamics at the end of
inflation are rich and a potential source of observational signatures.
Reheating, the energy transfer between the inflaton and Standard Model fields
(possibly through intermediaries) and their subsequent thermalization, can
provide clues to how inflation fits in with known high-energy physics. We
provide an overview of our current understanding of the nonperturbative,
nonlinear dynamics at the end of inflation, some salient features of realistic
particle physics models of reheating, and how the universe reaches a thermal
state before BBN. In addition, we review the analytical and numerical tools
available in the literature to study preheating and reheating and discuss
potential observational signatures from this fascinating era.
