  Quasi-periodic oscillations (QPOs) have been discovered in the X-ray emission
of many neutron stars and black holes. The QPOs with frequencies greater than
about 300 Hz are thought to be produced near the surfaces of neutron stars and
the event horizons of black holes. I first summarize some of the most important
properties of the QPOs seen in neutron star and black hole systems. I then
review some of the models that have been proposed and compare them with
observational data. Finally, I describe how these QPOs can be used to determine
the properties of dense matter and strong gravitational fields.
