Observations of the redshifted 21-cm line can probe the evolution of the intergalactic medium even prior widespread reionization. In this contribution I briefly review the status of 21-cm observations and what they start telling us about the evolution of the InterGalactic Medium at high redshift ($z > 6$).
How to cite
Metadata are provided both in "article" format (very similar to INSPIRE) as this helps creating
very compact bibliographies which can be beneficial to authors and
readers, and in "proceeding" format
which is more detailed and complete.