  Observations of thermonuclear (Type I) X-ray bursts from neutron stars in low
mass X-ray binaries (LMXB) with the Rossi X-ray Timing Explorer (RXTE) have
revealed large amplitude, high coherence X-ray brightness oscillations with
frequencies in the 300 - 600 Hz range. Substantial spectral and timing evidence
point to rotational modulation of the X-ray burst flux as the cause of these
oscillations, and it is likely that they reveal the spin frequencies of neutron
stars in LMXB from which they are detected. Here I review the status of our
knowledge of these oscillations and describe how they can be used to constrain
the masses and radii of neutron stars as well as the physics of thermonuclear
burning on accreting neutron stars.
