Observations of the 21 cm line at high redshifts promises to be a great probe of the early Universe during the Reionization and pre-Reionization epochs. I will describe how this signal is related to the underlying Astrophysics and Cosmology and show new, fast, large volume simulations of the 21 cm brightness temperature capable of generating the large field of view of the next generation of radio telescopes. The simulation extends to high redshifts (z≈30) and includes the effects of the Lyman alpha and collisional coupling as well as X-ray heating and corrections due to the gas bulk velocities. I will then describe how low frequency radio-interferometers now in construction should be able to probe the later stages of Reionization, even taking into account the huge foreground contamination. Moreover, at higher redshifts (z≈20), the Lyman α radiation emitted from the very first galaxies in the Universe makes a strong contribution to the 21cm signal on large scales and an SKA type experiment should be able to measure this signal, therefore making it probably the only telescope capable of giving us detailed information about the radiation emitted by the first stars and characterizing their host galaxies.