A conceptual design is presented for the Stratospheric Aerosol and Gas Experiment III (SAGE III) instrument which is currently being proposed for the Earth Observing System (Eos). SAGE III is designed to monitor globally the vertical distribution of stratospheric aerosols, ozone, water vapor, nitrogen dioxide and temperature by measuring the extinction and scattering of solar radiation in the 0.3 to 1.6 μm range through the earth's atmosphere. Solar radiation is reflected from a flat scanning mirror into a Cassegrain telescope, which forms an image on the entrance aperture of a grating spectrometer. The aspheric grating forms a stigmatic, flat field image on a silicon CCD array which serves as the detector for eight of the SAGE III channels. An interference filter in front of the array sorts the dispersed energy so that the desired order (either first, second, or third) reaches the focal plane. Pixels are grouped electronically to form the desired spectral bandpass of each channel. The ninth channel is taken from the grating zero order and uses a InGaAs PIN photodiode detector. A two axis gimbal system is capable of tracking (in azimuth) and scanning (in elevation) the sun through the earth's atmosphere when in the solar occultation mode, and then performing earth limb scans between occultation events. Data is sampled at 64 times per second and digitized to 14 bit resolution. Wavelength and radiance calibrations can be performed on-orbit. SAGE III has evolved from the highly successful Stratospheric Aerosol Measurement II (SAM II), SAGE, and SAGE II programs.