  The high time resolution and large area of the Rossi X-ray Timing Explorer
have been essential in the detection and characterization of high-frequency
quasi-periodic variability in the flux from neutron stars in low-mass X-ray
binaries. An unknown phenomenon prior to RXTE, kilohertz quasi-periodic
oscillations (QPOs) have now been detected from more than twenty systems. Their
high frequencies (up to 1330 Hz) imply that they are generated close to the
neutron star, where general relativistic effects are expected to play an
important role. I summarize current models for the kilohertz QPO phenomenon. In
particular, I show that there is a significant domain of agreement among the
models that can be used to constrain neutron star structure and look for
signatures of highly curved spacetime in the properties of the QPOs.
