Gas lasers provide fixed-wavelength output at a variety of wavelengths; the discrete wavelength produced depends on the identity of the gain medium. For example, He-Ne lasers produce strong laser lines near 633 nm, 1.15 microns, and 3.39 microns. Gas lasers can be used in fixed-wavelength, direct absorption diagnostics to measure species concentrations and can also be used in scattering experiments. In our laboratory, gas lasers have been employed to measure various hydrocarbon concentrations and also to investigate particle-laden flows. Gas-laser absorption diagnostics are arranged in the same general direct absorption setup as for QCLs and TDLs.