The CRISTA experiment (CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere) measured the global distribution of infrared limb emission spectra during 2 Space Shuttle missions in November 1994 and August 1997. Among other things, CRISTA was able to measure the emissions of carbon dioxide, ozone and atomic oxygen in the mesosphere and lower thermosphere. These species are of special interest, because they play an important role in the energy budget of the upper atmosphere. The CO₂ 4.3 micron daytime-radiances have been inverted to CO₂ number densities in the 60-130 km altitude range by using a non-local thermodynamic equilibrium model. A detailed sensitivity study of retrieved CO₂ was carried out. The resultant CO₂ volume mixing ratio (vmr) deviates from the constant value (360 ppm) between 70 and 80 km. This is approximately 10 km lower than indicated by model calculations. The decrease of the CO₂ vmr in the upper mesosphere is caused by molecular diffusion, which is of similar magnitude to turbulent diffusion at these altitudes. Atmospheric models are able to reproduce the CRISTA CO₂ profiles if the vertical eddy diffusion coefficient is reduced by a factor of 5-10. The global distribution of CRISTA-2 CO₂ density shows significant longitudinal and latitudinal structures which are compared with results obtained by the TIME-GCM, showing a very good agreement for the latitudinal distribution which represents the basic patterns of the global circulation. Ozone 9.6 micron infrared radiances were inverted to O₃ number densities in the 50-95 km altitude range. The ozone abundance profiles indicate vmrs of 1-2 ppmv at the stratopause, 0.5 ppmv or less around 80 km and typically 1 ppmv during daytime and 10 ppmv during night-time at the secondary maximum. The global distribution of upper mesospheric ozone shows significant latitudinal gradients and an enhancement in the equatorial upper mesosphere. ROSE-2 3D-model results indicate that the latitudinal gradients are strongly influenced by solar tides. At the polar night terminator a third ozone maximum is observed around 72 km, which is caused by a shift of the photochemical equilibrium between ozone-producing and destructing processes due to the low solar zenith angles. CRISTA performed the first global measurement of atomic oxygen 63 micron-radiation. Atomic oxygen densities were retrieved in the 130-175 km altitude regime. The resultant densities are approximately 40% lower than the predictions of the MSIS model for the low solar activity conditions of the CRISTA experiment. Radar measurements in the middle thermosphere show similar results and indicate that the atomic oxygen densities depend much more heavily on solar activity than previously assumed.