Neutral hydrogen plays a central role in both driving and regulating star formation over cosmic time. We will produce the first HI deep field, to be carried out with the VLA in B array and covering a redshift range from z=0 to z=0.45. The field is centered at the COSMOS field. We will produce HI images of at least 300 galaxies spread over the entire redshift range. With help from the already existing ancillary data we will get integrated profiles of about 500 spectroscopically known galaxies and with the use of stacking we will be able to determine mean HI properties as function of redshift, galaxy mass, color and location in the large scale structure. This will be the first HI imaging that goes well beyond a redshift of z=0.2, doubling the lookback time of HI imaging studies. Ancillary benefits will include deep continuum imaging, HI absorption, and a search for transients.