The early history of our observable universe and its development are based on two major poles: cosmic inflation and big-bang nucleosynthesis (BBN). The inflation successfully creates the homogeneous, isotropic, and spatially flat universe, and BBN successfully produces all the light elements with correct abundance values. These two phenomena happen at two widely separated energy scales with inflation at around 10¹⁶ GeV and BBN at around 10 MeV. However, except in some phenomenological studies, not much physics is understood of the stage which occurs between the two, mainly because of the lack of observations. Inflation creates exponentially large empty space contained with only homogeneous inflaton fields. Hence, the natural physical process that could occur is the transfer of inflaton energy into the standard model fields, which can subsequently set the initial condition for BBN. The phase where this energy transfer from inflaton to radiation happens is called reheating. How the radiation fields are coupled with the inflaton, how the produced radiation is thermalized, and how long the decay processes last are some of the central questions that are still the active areas of research in reheating. The lack of experimental observations makes this phase essentially unconstrained. However, there are many attempts to constrain the reheating phase through some indirect cosmological observables such as primordial gravitational wave, primordial magnetic field, dark matter abundance, and cosmic microwave background (CMB) anisotropy, and these are the main topics of discussion in my talk.