  The physical origin of high-frequency QPOs (HFQPOs) in black-hole X-ray
binaries remains an enigma despite many years of detailed observational
studies. Although there exists a number of models for HFQPOs, many of these are
simply "notions" or "concepts" without actual calculation derived from fluid or
disk physics. Future progress requires a combination of numerical simulations
and semi-analytic studies to extract physical insights. We review recent works
on global oscillation modes in black-hole accretion disks, and explain how,
with the help of general relativistic effects, the energy stored in the disk
differential rotation can be pumped into global spiral density modes in the
disk, making these modes grow to large amplitudes under certain conditions
("corotational instability"). These modes are robust in the presence of disk
magnetic fields and turbulence. The computed oscillation mode frequencies are
largely consistent with the observed values for HFQPOs in BH X-ray binaries.
The approximate 2:3 frequency ratio is also expected from this model. The
connection of HFQPOs with other disk properties (such as production of episodic
jets) is also discussed.
