SCIAMACHY is a spectrometer designed to measure sunlight transmitted, reflected, and scattered by the Earth's atmosphere or surface in the ultraviolet, visible, and near-infrared wavelength range (240–2380 nm) at moderate spectral resolution (0.2–1.5 nm, λ/Δλ ≈ 1000–10,000). It measures Earthshine radiance in limb and nadir viewing geometries and solar or lunar light transmitted through the atmosphere observed in occultation. The extraterrestrial solar irradiance and lunar radiance are determined from observations of the sun and moon above the atmosphere. The absorption, reflection, and scattering behavior of the atmosphere and Earth's surface is determined by comparing Earthshine radiance and solar irradiance. Inversion of the ratio of Earthshine radiance and solar irradiance yields information about the amounts and distribution of important atmospheric constituents and the spectral reflectance (or albedo) of the Earth's surface. SCIAMACHY was conceived to improve knowledge and understanding of issues important for the chemistry and physics of the Earth's atmosphere (troposphere, stratosphere, and mesosphere) and potential changes resulting from increasing anthropogenic activity or natural variability. Topics include tropospheric pollution, troposphere-stratosphere exchange processes, stratospheric ozone chemistry, and solar variability and special events like volcanic eruptions. Inversion of SCIAMACHY measurements enables determination of atmospheric constituents such as O3, O2, O2(Δ), O4, BrO, OClO, ClO, SO2, H2CO, NO, NO2, NO3, CO, CO2, CH4, H2O, N2O, and aerosol, as well as parameters like pressure, temperature, radiation field, cloud cover, cloud-top height, and surface spectral reflectance. A special feature is the combined limb-nadir measurement mode. SCIAMACHY is part of the atmospheric chemistry payload onboard ENVISAT. It measures scattered and reflected spectral radiance in nadir and limb geometry, spectral radiance transmitted through the atmosphere in solar and lunar occultation geometry, and extraterrestrial solar irradiance and lunar radiance. Limb, nadir, and occultation measurements are planned during every orbit. Trace gases, aerosols, clouds, and the Earth's surface modify the light observed by SCIAMACHY via absorption, emission, and scattering processes. Inversion of radiance and irradiance measurements enables retrieval of the amounts and distributions of significant constituents from their spectral signatures. SCIAMACHY and GOME, a smaller version of SCIAMACHY, represent a new generation of space-based remote sounding sensors. Using data from GOME, the feasibility of the instrument and retrieval concepts have been successfully demonstrated for nadir observations. The trace gases O3, NO2, BrO, OClO, SO2, andSCIAMACHY is a spectrometer designed to measure sunlight transmitted, reflected, and scattered by the Earth's atmosphere or surface in the ultraviolet, visible, and near-infrared wavelength range (240–2380 nm) at moderate spectral resolution (0.2–1.5 nm, λ/Δλ ≈ 1000–10,000). It measures Earthshine radiance in limb and nadir viewing geometries and solar or lunar light transmitted through the atmosphere observed in occultation. The extraterrestrial solar irradiance and lunar radiance are determined from observations of the sun and moon above the atmosphere. The absorption, reflection, and scattering behavior of the atmosphere and Earth's surface is determined by comparing Earthshine radiance and solar irradiance. Inversion of the ratio of Earthshine radiance and solar irradiance yields information about the amounts and distribution of important atmospheric constituents and the spectral reflectance (or albedo) of the Earth's surface. SCIAMACHY was conceived to improve knowledge and understanding of issues important for the chemistry and physics of the Earth's atmosphere (troposphere, stratosphere, and mesosphere) and potential changes resulting from increasing anthropogenic activity or natural variability. Topics include tropospheric pollution, troposphere-stratosphere exchange processes, stratospheric ozone chemistry, and solar variability and special events like volcanic eruptions. Inversion of SCIAMACHY measurements enables determination of atmospheric constituents such as O3, O2, O2(Δ), O4, BrO, OClO, ClO, SO2, H2CO, NO, NO2, NO3, CO, CO2, CH4, H2O, N2O, and aerosol, as well as parameters like pressure, temperature, radiation field, cloud cover, cloud-top height, and surface spectral reflectance. A special feature is the combined limb-nadir measurement mode. SCIAMACHY is part of the atmospheric chemistry payload onboard ENVISAT. It measures scattered and reflected spectral radiance in nadir and limb geometry, spectral radiance transmitted through the atmosphere in solar and lunar occultation geometry, and extraterrestrial solar irradiance and lunar radiance. Limb, nadir, and occultation measurements are planned during every orbit. Trace gases, aerosols, clouds, and the Earth's surface modify the light observed by SCIAMACHY via absorption, emission, and scattering processes. Inversion of radiance and irradiance measurements enables retrieval of the amounts and distributions of significant constituents from their spectral signatures. SCIAMACHY and GOME, a smaller version of SCIAMACHY, represent a new generation of space-based remote sounding sensors. Using data from GOME, the feasibility of the instrument and retrieval concepts have been successfully demonstrated for nadir observations. The trace gases O3, NO2, BrO, OClO, SO2, and