The cryogenic limb array etalon spectrometer (CLAES) is one of 10 experiments launched in September 1991 on the NASA Upper Atmosphere Research Satellite (UARS). CLAES measures altitude profiles of temperature, pressure, O3, H2O, CH4, N2O, NO, NO2, N2O5, HNO3, ClONO2, HCl, CFC 11, CFC 12, and aerosol absorption coefficients. These data are obtained between 10 and 60 km with 2.5-km vertical resolution and 500-km horizontal grid size and between latitudes 80¿ north and south. Since CLAES actually measures infrared spectral earthlimb emissions, it can operate continuously throughout the diurnal cycle. The on-orbit lifetime as dictated by stored cryogens which cool optics and detectors is estimated to be 21 months. The experiment will perform the first global mapping of stratospheric ClONO2, CFC 11, CFC 12, and N2O5, and these data, along with the simultaneous measurement of temperature and the other constituents listed above, should contribute to a significant improvement in our understanding of stratospheric and mesospheric photochemistry, radiative structure, and dynamics. CLAES began viewing the atmosphere in early October 1991, and the first several months of observations will be discussed. Examples of atmospheric spectral emission profiles for a number of constituents are presented as well as responsivity and noise parameters. These data show the instrument performance to be excellent and close to prelaunch predictions. An overview of the experiment and instrumentation is presented, various scientific observational modes are described, and the algorithms and software used to retrieve atmospheric parameters from emission spectra are discussed. ¿ American Geophysical Union 1993