Exploiting the huge collecting area of the Arecibo 305 meter antenna, the
ALFALFA HI 21 cm line survey has mapped 7000 square degrees of the high Galactic
latitude sky out to z~0.06, cataloging ~30,000 HI sources. Virtually all star-forming
galaxies contain atomic gas; in this talk, I will review the relationship between
the optically- and HI-selected 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. I will also give an overview of planned SKA pathfinder surveys
which will explore the cosmic evolution of properties of the HI population.