Spectrometers are used to determine the chemical composition and temperature of the atmosphere, exhausts, and gas releases. A relevant part of the electromagnetic spectrum is detected via absorption spectroscopy by means of artificial (e. g., lamps and lasers) or natural light sources (e. g., the Sun), as well as via emission spectroscopy using temperature differences or atmospheric radiation. The currently preferred spectrometer techniques are Fourier transform infrared spectroscopy (FTIRFourier-TransformInfrared Spectroscopy (FTIR)) in the infrared (IR) and visible (VIS), differential optical absorption spectroscopy (DOASdifferentialoptical absorption spectroscopy (DOAS)) in the VIS and ultraviolet (UV), as well as microwave radiometry (MWRmicrowaveradiometry (MWR)). To achieve high sensitivity, tuned or broadband lasers are employed in laser spectrometers. Radiation absorption and scattering can be detected using spectrometers by an open-path configuration either through a single pass or multiple passes by steering optics in the atmosphere or exhausts. Analysis of the measured spectral features can be used to detect atmospheric gas concentrations, temperature, pressure, as well as aerosol scattering and absorption. The basic theory with equations and retrieval algorithms for processing measured spectra is described. In addition to characteristic parameters, quality assurance and quality control, calibration, and the necessary maintenance associated with the different measurement principles are presented.