The cosmological evolution of neutral gas mass, Omega_HI, provide
information on the gas reservoir for star formation and accretion
phenomena on global scales. Measurements at z>2 based on large surveys
of quasar absorbers observed at optical wavelengths now provide
reliable estimates of this quantity. Observations at z~0 of HI
emission of galaxies at radio wavelength also offer a strong
constraint on Omega_HI, with a value considerably lower than measured
at high-redshifts. ASKAP, MeerKAT (and ultimately SKA) offer prospects
to derive this quantity at z>0 with direct emission detections,
stacking and blind absorption techniques, thus providing important
clues on the evolution of Omega_HI in a so-far little-probed time
range.