Neutral hydrogen at high redshift: probing structure formation
  Large inhomogeneities in neutral hydrogen in the universe can be detected at redshifts $z \leq 10$ using the redshifted 21cm line emission from atomic hydrogen. This paper reviews the expected evolution of neutral hydrogen and presents estimates for future surveys of HI at $z \approx 3$. We also discuss the possibility of detecting neutral hydrogen at higher redshifts. 
J.S.Bagla (Institute of Astronomy, Cambridge)