The Halogen Occultation Experiment (HALOE) was launched on the Upper Atmosphere Research Satellite (UARS) in September 1991 to study the middle atmosphere, focusing on ozone distribution and processes affecting ozone levels. The experiment uses solar occultation to measure vertical profiles of various gases, including O3, HCl, HF, CH4, H2O, NO, NO2, aerosol extinction, and temperature. The measurements cover latitudes from 80°S to 80°N over a year, with extensive observations of the Antarctic region during spring. The altitude range of measurements extends from about 15 km to 60–130 km. The experiment has been successfully operated, with all performance criteria met or exceeded. Internal data consistency checks, comparisons with correlative measurements, and qualitative comparisons with 1985 atmospheric trace molecule spectroscopy (ATMOS) results show good agreement. Key findings include the utility of CH4, HF, and H2O as tracers, 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 experiment's sensitivity is about one order of magnitude better than that of the MAPS instrument, necessary for sounding tenuous gases in the middle atmosphere. The overall scientific goal is to provide global-scale data on temperature, ozone, odd chlorine (ClOy), odd nitrogen (NOy), and odd hydrogen (HOy) compounds to study their chemistry and dynamics processes. The paper describes the measurement approach, geographic coverage, instrument and performance, data processing, inversion approach, data validation studies, and initial measurement results.The Halogen Occultation Experiment (HALOE) was launched on the Upper Atmosphere Research Satellite (UARS) in September 1991 to study the middle atmosphere, focusing on ozone distribution and processes affecting ozone levels. The experiment uses solar occultation to measure vertical profiles of various gases, including O3, HCl, HF, CH4, H2O, NO, NO2, aerosol extinction, and temperature. The measurements cover latitudes from 80°S to 80°N over a year, with extensive observations of the Antarctic region during spring. The altitude range of measurements extends from about 15 km to 60–130 km. The experiment has been successfully operated, with all performance criteria met or exceeded. Internal data consistency checks, comparisons with correlative measurements, and qualitative comparisons with 1985 atmospheric trace molecule spectroscopy (ATMOS) results show good agreement. Key findings include the utility of CH4, HF, and H2O as tracers, 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 experiment's sensitivity is about one order of magnitude better than that of the MAPS instrument, necessary for sounding tenuous gases in the middle atmosphere. The overall scientific goal is to provide global-scale data on temperature, ozone, odd chlorine (ClOy), odd nitrogen (NOy), and odd hydrogen (HOy) compounds to study their chemistry and dynamics processes. The paper describes the measurement approach, geographic coverage, instrument and performance, data processing, inversion approach, data validation studies, and initial measurement results.