The methodology and validation of upper tropospheric humidity (UTH) measurements by the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) are described. The MLS instrument was not specifically designed to measure UTH, but its measurement is achieved by modeling the radiance signal from a window channel as the instrument field of view is scanned through the troposphere. UTH is retrieved as a piecewise-linear function in relative humidity with respect to ice (% RHi) with breakpoints at 147, 215, 316, and 464 hPa. MLS version 4.9 is believed to be the most accurate version to date; its estimated global-averaged accuracy (precision) is 23(21), 22(10), 22(8), and 50(19) %RHi at 147, 215, 316, and 464, hPa respectively. The validation of UTH is established by its consistency with (1) climatological fields, (2) coincident comparisons with in situ Vaisala RS80 balloon sondes, frostpoint hygrometers and Measurement of Ozone and Water Vapor by Airbus In-Service Aircraft measurements, (3) cirrus observations, and (4) meteorological dynamics and assimilation fields. MLS climatological UTH is morphologically consistent with Stratospheric Aerosol and Gas Experiment II and Vaisala zonal climatologies. Comparisons with coincident in situ data show a wide range of agreement. Relative to the other in situ instruments, V4.9 MLS humidities agree on average within 11% at 147 hPa, 3--36% dry at 215 hPa, and 14--39% dry at 316 and 464 hPa. The standard deviation of the individual comparisons about the mean between MLS UTH and the other techniques is approximately 25% RHi at all heights. Humidity measurements in cirrus are compared with UARS Cryogenic Limb Array Etalon Spectrometer (CLAES) extinction measurements and an MLS cirrus detection algorithm. CLAES extinction measurements from 316 to 147 hPa which indicate the presence of cirrus correspond well with MLS UTH measurements greater than ~100 %RHi. Global MLS UTH distributions appear consistent with those expected from upper tropospheric dynamics and with the Goddard Data Assimilation Office humidity fields. ¿ 2001 American Geophysical Union