HALOE is an optical remote sensor that measures extinction of solar radiation caused by the Earth's atmosphere in eight channels ranging in wavelength from 2.5 to 10.1 micrometers. These measurements, which occur twice each satellite orbit during solar occultation, are inverted to yield vertical distributions of middle atmosphere ozone (03), water vapor (H20) , nitrogen dioxide (NO2) , nitric oxide (NO) , hydrogen fluoride (HF), hydrogen chloride (HC1), and methane (CH4). A channel located in the 2.7 micrometers region is used to infer the tangent point pressure by measuring carbon dioxide (CO2) absorption. The HALOE instrument consists of a two-axis gimbal system, telescope, spectral discrimination optics and a 12-bit data system. The gimbal system tracks the solar radiometric centroid in the azimuthal plane and tracks the solar limb in the elevation plane placing the instrument's instantaneous field-of-view 4 arcminutes down from the solar top edge. The instrument gathers data for tangent altitudes ranging from 150 km to the Earth's horizon. Prior to an orbital sunset and after an orbital sunrise, the HALOE automatically performs calibration sequences to enhance data interpretation. The instrument is presently being tested at the Langley Research Center in preparation for launch on the Upper Atmosphere Research Satellite near the end of this decade. This paper describes the instrument design, operation, and functional performance.