Before the launch of the Rossi X‐ray Timing Explorer (RXTE) it was recognized that neutron star accretion disks could extend inward to very near the neutron star surface, and thus be governed by millisecond timescales. Previous missions lacked the sensitivity to detect them. The kilohertz quasi‐periodic oscillations (QPO) thatRXTEdiscovered are often, but not always, evident in the X‐ray flux. In 8 yearsRXTEhas found kilohertz signals in about a fourth of 100 low‐mass X‐ray binaries (LMXB) containing neutron stars. The observed power spectra have simple dominant features, the two kilohertz oscillations, a low frequency oscillation, and band‐limited white noise. They vary systematically with changes in other source properties and offer the possibility of comparison with model predictions. New information from the millisecond pulsars resolves some questions about the relations of the QPO and the spin. Coherence, energy spectrum and time lag measurements have indicated systematic behaviors, which should constrain mechanisms. © 2004 American Institute of Physics