The Limb Infrared Monitor of the Stratosphere experiment was launched on the NIMBUS 7 spacecraft to test the capabilities of infrared limb scanning radiometry to sound the composition and structure of the middle atmosphere. The instrument, a six channel radiometer, operated from October 25, 1978, to May 28, 1979, returning each day more than 7000 profiles of radiance as a function of vertical position across the limb. These radiance were subsequently inverted to determine temperature, ozone, water vapor, nitric acid, and nitrogen dioxide distributions in the stratosphere and mesosphere. This paper describes the experiment and its results. The measurement requirements are discussed, and critical instrument parameters are identified. Next, the instrument ad its laboratory calibration are described. In orbit, its operation was stable and nearly flawless. The operational data reduction involved calibrating the radiances, performing a nonlinear inversion based on the emissivity growth approximation, and mapping the data. The methods of validating the data through simulation, internal checks, and intercomparisons are summarized, and conservative estimates for accuracy and precision are obtained. Temperature accuracy and precision are estimated to be 2 K and ~0.4 K, respectively. Accuracy of the trace gases is ~25%, comparable to the etimatd accuray of the in situ comparison data; their precision is ~0.25 pmv (O3 and H2O) and 0.2 pbv (HNO3 and NO2). Examples of the vertical profiles, maps and cross sections of the data show previously unobserved variations with latitude, altitude, and time. The results demonstrate that infrared limb scanning is an extremely powerful method for sounding the middle atmosphere.