  One of the major science goals of the Square Kilometre Array (SKA) is to
understand the role played by atomic hydrogen (HI) gas in the evolution of
galaxies throughout cosmic time. The hyperfine transition line of the hydrogen
atom at 21-cm is one of the best tools to detect and study the properties of HI
gas associated with galaxies. In this article, we review our current
understanding of HI gas and its relationship with galaxies through observations
of the 21-cm line both in emission and absorption. In addition, we provide an
overview of the HI science that will be possible with SKA and its pre-cursors
and pathfinders, i.e. HI 21-cm emission and absorption studies of galaxies from
nearby to high redshifts that will trace various processes governing galaxy
evolution.
