The Halogen Occultation Experiment (HALOE) was launched on the Upper Atmosphere Research Satellite (UARS) on September 12, 1991, and began science observations on October 11. The experiment uses solar occultation to measure vertical profiles of O3, HCl, HF, CH4, H2O, NO, NO2, aerosol extinction, and temperature versus pressure. It provides global coverage from 80°S to 80°N, with extensive observations of the Antarctic region during spring. The altitude range of measurements extends from about 15 km to 60-130 km, depending on channel. The experiment has been essentially flawless, with all performance criteria meeting or exceeding specifications. Internal data consistency checks, comparisons with correlative measurements, and qualitative comparisons with 1985 atmospheric trace molecule spectroscopy (ATMOS) results are in good agreement. Examples of pressure versus latitude cross sections and a global orthographic projection for the September 21 to October 15, 1992, period show the utility of CH4, HF, and H2O as tracers, the occurrence of dehydration in the Antarctic lower stratosphere, the presence of the water vapor hygropause in the tropics, evidence of Antarctic air in the tropics, the influence of Hadley tropical upwelling, and the first global distribution of HCl, HF, and NO throughout the stratosphere. Nitric oxide measurements extend through the lower thermosphere. The HALOE experiment was conceived to provide critical data for study of the ozone distribution and those processes which affect ozone levels. The experiment uses the principle of satellite solar occultation to sound the stratosphere, mesosphere, and lower thermosphere. Using this technique, absorption of solar energy in selected spectral bands is used to infer vertical profiles of temperature, pressure, and mixing ratios of key gases involved in the ozone chemistry. The HALOE instrument includes both broadband and gas filter channels covering selected portions of the spectral range from 2.45 μm to 10.04 μm. The experiment was launched onboard the UARS spacecraft by the Space Shuttle Discovery on September 12, 1991. A science team (authors of this paper) was formed to guide the experiment development, assist in flight data validation, and carry out scientific investigations. The HALOE experiment was first proposed in 1975 shortly after the initial paper was published by Molina and Rowland [1974] discussing the possible effects of chlorofluorocarbons (CFCs) on atmospheric ozone. The essentials of the gas filter technique used in the occultation mode have been described by Russell et al. [1977]. The technique builds on the same general method employed by the Monitoring of Air Pollution by Satellite (MAPS) experiment [ReichThe Halogen Occultation Experiment (HALOE) was launched on the Upper Atmosphere Research Satellite (UARS) on September 12, 1991, and began science observations on October 11. The experiment uses solar occultation to measure vertical profiles of O3, HCl, HF, CH4, H2O, NO, NO2, aerosol extinction, and temperature versus pressure. It provides global coverage from 80°S to 80°N, with extensive observations of the Antarctic region during spring. The altitude range of measurements extends from about 15 km to 60-130 km, depending on channel. The experiment has been essentially flawless, with all performance criteria meeting or exceeding specifications. Internal data consistency checks, comparisons with correlative measurements, and qualitative comparisons with 1985 atmospheric trace molecule spectroscopy (ATMOS) results are in good agreement. Examples of pressure versus latitude cross sections and a global orthographic projection for the September 21 to October 15, 1992, period show the utility of CH4, HF, and H2O as tracers, the occurrence of dehydration in the Antarctic lower stratosphere, the presence of the water vapor hygropause in the tropics, evidence of Antarctic air in the tropics, the influence of Hadley tropical upwelling, and the first global distribution of HCl, HF, and NO throughout the stratosphere. Nitric oxide measurements extend through the lower thermosphere. The HALOE experiment was conceived to provide critical data for study of the ozone distribution and those processes which affect ozone levels. The experiment uses the principle of satellite solar occultation to sound the stratosphere, mesosphere, and lower thermosphere. Using this technique, absorption of solar energy in selected spectral bands is used to infer vertical profiles of temperature, pressure, and mixing ratios of key gases involved in the ozone chemistry. The HALOE instrument includes both broadband and gas filter channels covering selected portions of the spectral range from 2.45 μm to 10.04 μm. The experiment was launched onboard the UARS spacecraft by the Space Shuttle Discovery on September 12, 1991. A science team (authors of this paper) was formed to guide the experiment development, assist in flight data validation, and carry out scientific investigations. The HALOE experiment was first proposed in 1975 shortly after the initial paper was published by Molina and Rowland [1974] discussing the possible effects of chlorofluorocarbons (CFCs) on atmospheric ozone. The essentials of the gas filter technique used in the occultation mode have been described by Russell et al. [1977]. The technique builds on the same general method employed by the Monitoring of Air Pollution by Satellite (MAPS) experiment [Reich