The JPL MkIV balloon interferometer measures the composition of the Earth’s atmosphere remotely from a stratospheric balloon or from the ground. It observes the sunlight transmitted by the atmosphere at infrared wavelengths. The absorption of the sunlight by trace gases at specific wavelengths provides information on the abundance of these trace gases. When deployed from a balloon, observations at sunset and sunrise provide vertical profiles of trace gas abundances at high vertical resolution. The MkIV interferometer has been deployed on more than 20 balloon flights at mid-latitudes and in the Arctic since 1989. On each flight over 40 trace gases are measured simultaneously, providing a detailed inventory of the composition of Earth's atmosphere and its development over time.