  We review the results of HI line surveys of extragalactic sources in the
local Universe. In the last two decades major efforts have been made in
establishing on firm statistical grounds the properties of the HI source
population, the two most prominent being the HI Parkes All Sky Survey (HIPASS)
and the Arecibo Legacy Fast ALFA survey (ALFALFA). We review the choices of
technical parameters in the design and optimization of spectro-photometric
"blind" HI surveys, which for the first time produced extensive HI-selected
data sets. Particular attention is given to the relationship between optical
and HI populations, the differences in their clustering properties and the
importance of HI-selected samples in contributing to the understanding of
apparent conflicts between observation and theory on the abundance of low mass
halos. The last section of this paper provides an overview of currently ongoing
and planned surveys which will explore the cosmic evolution of properties of
the HI population.
