Leo Blitz (1), Andrew West (1), Erik Rosolowsky (2)
(1) University of California, Berkeley
(2) Harvard-Smithsonian Center for Astrophysics

We review the problem of gas depletion times in spiral galaxies: the molecular gas must be replaced in about 10$^9$ years if we are not in the last throes of star formation. We show that the radial gas distributions in spiral galaxies are sufficiently similar to one another that the gas is probably being resupplied in some manner. Infall from High Velocity Clouds and their extragalactic analogues, as well as the failure to detect inflows through the disk, suggest that some other mechanism is likely to be responsible for gas redistribution. We speculate that this mechanism might be a modified form of galaxy harassment and propose some observational tests.