The theory of reheating after inflation is developed, showing that the classical inflaton field typically decays rapidly into bosons due to parametric resonance, followed by decay into other particles and eventual thermalization. Complete reheating requires the inflaton to decay into other particles, imposing constraints on inflationary models and suggesting the inflaton could be a dark matter candidate. Reheating occurs in three stages: pre-heating, where the inflaton decays into bosons; decay of these particles; and thermalization. Pre-heating is characterized by exponential growth of particle numbers due to resonance, leading to rapid decay of the inflaton field. The reheating temperature depends on the decay rates and is estimated as $ T_r \simeq 0.1 \sqrt{\Gamma M_p} $. In models with spontaneous symmetry breaking, the inflaton field decays into bosons and fermions, with the effective mass of the field increasing due to interactions. The process of pre-heating is crucial as it leads to an extremely rapid decay of the inflaton field, which can significantly affect the reheating temperature and the subsequent thermalization of particles. The results show that the reheating temperature can be much higher than previously estimated, and that pre-heating plays a vital role in the cosmological scenario, influencing the equation of state and the production of primordial black holes. The study highlights the importance of parametric resonance in the early universe and the implications for inflationary models and dark matter.The theory of reheating after inflation is developed, showing that the classical inflaton field typically decays rapidly into bosons due to parametric resonance, followed by decay into other particles and eventual thermalization. Complete reheating requires the inflaton to decay into other particles, imposing constraints on inflationary models and suggesting the inflaton could be a dark matter candidate. Reheating occurs in three stages: pre-heating, where the inflaton decays into bosons; decay of these particles; and thermalization. Pre-heating is characterized by exponential growth of particle numbers due to resonance, leading to rapid decay of the inflaton field. The reheating temperature depends on the decay rates and is estimated as $ T_r \simeq 0.1 \sqrt{\Gamma M_p} $. In models with spontaneous symmetry breaking, the inflaton field decays into bosons and fermions, with the effective mass of the field increasing due to interactions. The process of pre-heating is crucial as it leads to an extremely rapid decay of the inflaton field, which can significantly affect the reheating temperature and the subsequent thermalization of particles. The results show that the reheating temperature can be much higher than previously estimated, and that pre-heating plays a vital role in the cosmological scenario, influencing the equation of state and the production of primordial black holes. The study highlights the importance of parametric resonance in the early universe and the implications for inflationary models and dark matter.