Millisecond oscillations in X-ray binaries

The first millisecond X-ray variability phenomena from accreting compact ob jects have recently been discovered with the Rossi X-ray Timing Explorer Th ree new phenomena are observed from low-mass X-ray binaries containing low- magnetic-field neutron stars: millisecond pulsations, burst oscillations, a nd kilohertz quasi-periodic oscillations. Models for these new phenomena in volve the neutron star spin and orbital motion close around the neutron sta r, and rely explicitly on our understanding of strong gravity and dense mat ter. I review the observations of these new neutron-star phenomena and some possibly related phenomena in black-hole candidates, and describe the atte mpts to use these observations to perform measurements of fundamental physi cal interest in these systems.