2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/12057Large 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.8 pages LaTeX, uses paspconf.sty and epsf.sty. To appear in "Highly Redshifted Radio Lines", eds. C. Carilli, S. Radford, K. Menten, G. Langston, (PASP: San Francisco)Astrophysicstext