Several important areas of modern technology require fast, accurate and convenient measurement of trace species concentrations in gaseous environments. Examples include reactive plasma processing, pollutant monitoring and semiconductor manufacturing. The most general tool available for performing such measurements is absorption spectroscopy. However, this approach often lacks the requisite sensitivity for many applications. Cavity ringdown laser absorption spectroscopy is a technique that has emerged in the last several years for measuring very small light absorption by gaseous samples. The method can easily detect ~1 ppm fractional absorption by a gaseous sample in about 1 µs with very straightforward pulsed laser technology. (In fact, the technique is so simple that several universities now feature experiments in undergraduate teaching labs, even for measurements of free radicals in flames!) Higher sensitivity is possible, but with a greater degree of complexity in the measurement and apparatus. Moreover, the method works in the visible, UV and most recently in the IR.