  The variability properties of accreting compact objects predicted by
dynamical models are characterized by a number of distinct frequencies that are
specific to each model. Because the accretion disks around neutron stars and
black holes are hydrodynamic flows, these modulation frequencies cannot be
strictly equal to test particle frequencies. I discuss several implications of
hydrodynamic corrections to the modulation frequencies predicted by dynamical
models. Finally, I show that the recent detection of yet a third kHz QPO in
three neutron-star systems favors a previously developed model that attributes
the various QPO frequencies to fundamental general relativistic frequencies in
the accretion flow.
