  The recently initiated Arecibo Legacy Fast ALFA (ALFALFA) survey aims to map
7000 square degrees of the high galactic latitude sky visible from Arecibo,
providing a HI line spectral database covering the redshift range between -1600
km/s and 18,000 km/s with 5 km/s resolution. Exploiting Arecibo's large
collecting area and small beam size, ALFALFA is specifically designed to probe
the faint end of the HI mass function in the local universe and will provide a
census of HI in the surveyed sky area to faint flux limits, making it
especially useful in synergy with wide area surveys conducted at other
wavelengths. ALFALFA will also provide the basis for studies of the dynamics of
galaxies within the Local and nearby superclusters, will allow measurement of
the HI diameter function, and enable a first wide-area blind search for local
HI tidal features, HI absorbers at z < 0.06 and OH megamasers in the redshift
range 0.16 < z < 0.25. Although completion of the survey will require some five
years, public access to the ALFALFA data and data products will be provided in
a timely manner, thus allowing its application for studies beyond those
targeted by the ALFALFA collaboration. ALFALFA adopts a two-pass, minimum
intrusion, drift scan observing technique which samples the same region of sky
at two separate epochs to aid in the discrimination of cosmic signals from
noise and terrestrial interference. Survey simulations, which take into account
large scale structure in the mass distribution and incorporate experience with
the ALFA system gained from tests conducted during its commissioning phase,
suggest that ALFALFA will detect on the order of 20,000 extragalactic HI line
sources out to z=0.06, including several hundred with HI masses of less than
10^{7.5} msun.
