The Microwave Limb Sounder (MLS) is a passive microwave radiometer/spectrometer that measures microwave thermal emission from the limb (edge) of Earth’s atmosphere to sense vertical profiles of atmospheric gases, temperature, pressure, and cloud ice. MLS measurements are acquired globally day and night and can be obtained in the presence of ice clouds and aerosols that prevent measurements by shorter-wavelength infrared, visible, and ultraviolet sensing techniques. MLS data support investigations in three general scientific areas: Stratospheric ozone layer stability, climate change, and air quality. Along with its installation aboard NASA’s Aura satellite, an MLS also was aboard NASA’s Upper Atmosphere Research Satellite (UARS) and collected data from 1991 to 2001.