ALFALFA is a blind HI survey with the Arecibo telescope that will map 7000 square degrees of high galactic latitude sky with unprecedented sensitivity and resolution. It will therefore strongly constrain the number density of dark galaxies: low-mass, HI-rich systems with little or no star formation that would alleviate the cosmological "missing satellite" problem. In this talk I will review the theoretical case for the existence of dark galaxies, and I'll discuss the nature of two optically dark HI complexes in the Virgo cluster in the context of ALFALFA and follow-up observations.