  The Square Kilometre Array (SKA) will be a formidable instrument for the
detailed study of neutral hydrogen (HI) in external galaxies and in our own
Galaxy and Local Group. The sensitivity of the SKA, its wide receiver bands,
and the relative freedom from radio frequency interference at the SKA sites
will allow the imaging of substantial number of high-redshift galaxies in HI
for the first time. It will also allow imaging of galaxies throughout the Local
Volume at resolutions of <100 pc and detailed investigations of galaxy disks
and the transition between disks, halos and the intergalactic medium (IGM) in
the Milky Way and external galaxies. Together with deep optical and
millimetre/sub-mm imaging, this will have a profound effect on our
understanding of the formation, growth and subsequent evolution of galaxies in
different environments. This paper provides an introductory text to a series of
nine science papers describing the impact of the SKA in the field of HI and
galaxy evolution. We propose a nested set of surveys with phase 1 of the SKA
which will help tackle much of the exciting science described. Longer commensal
surveys are discussed, including an ultra-deep survey which should permit the
detection of galaxies at z=2, when the Universe was a quarter of its current
age. The full SKA will allow more detailed imaging of even more distant
galaxies, and allow cosmological and evolutionary parameters to be measured
with exquisite precision.
