HI Velocity Dispersions and Flaring:Disk Masses and the Shape of Dark Matter Halos

I briefly review the use of measurements of the HI velocity dispersion and gas layer flaring in galaxy disks to determine the baryonic mass of the disks. I compare that to results from stellar dynamics. In systems with low-mass disks, flaring can also provide information on the flattening of the dark matter halos. New results from the thesis of Jess O'Brien are summarized, in particular an application to UGC 7321 that shows the dark matter halo in that galaxy to be close to spherical.