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Summary form only given, as follows. Cavity ring-down spectroscopy (CRDS) is newly developed spectroscopic technique which may be used to determine the concentration of very dilute gas-phase species present in hostile environments. Unlike traditional absorption techniques which measure the attenuation of light passing through a sample, in CRDS light from a pulsed laser is injected into a high-finesse cavity formed by two or more high reflectivity mirrors. Light leaking from the resonator through one of the mirrors is then used to measure the rate of decay of the pulse of light within the cavity. Absorption by species present within the resonator increase the rate of decay of light in the cavity. Therefore, by measuring this ring-down rate with and without sample present within the resonator, the absorbance of the sample may be measured and its absolute concentration determined. The largest advantage of CRDS is its insensitivity to fluctuations in the intensity of the light source; these fluctuations limit the sensitivity of traditional absorption techniques. Furthermore, since the decay of coherent light from the resonator is being detected, CRDS may be used to measure species in environments with high background radiation levels such as flames and discharges. Advancements in the use of this technique to measure the concentration of species in hostile environments will be discussed as well as the application of continuous-wave laser sources to the technique.