  This is a pedagogical introduction to early cosmology and the host of
fundamental physics involved in it (particle physics, grand unification and
general relativity). Inflation and the inflaton field are the central theme of
this review. The quantum field treatment of the inflaton is presented including
its out of equilibrium evolution and the use of nonperturbative methods. The
observational predictions for the CMB anisotropies are briefly discussed.
Finally, open problems and future perspectives in connection with dark energy
and string theory are overviewed.
