  The kilo-Hertz Quasi--Periodic Oscillations in X-ray binaries could originate
within the accretion flow, and be a signature of non--linear fluid oscillations
and mode coupling in strong gravity. The possibility to decipher these systems
will impact our knowledge of fundamental parameters such as the neutron star
mass, radius, and spin. Thus they offer the possibility to constrain the
nuclear equation of state and the rotation parameter of stellar--mass black
holes. We review the general properties of these oscillations from a
hydrodynamical point of view, when the accretion flow is subject to external
perturbations and summarize recent results.
