Absorption Spectroscopy with tunable diode lasers in the infrared has become a standard technique for the chemical analysis of gases . Some of the advantages offered by this technique are specificity, sensitivity and fast response. Tunable Diode Laser Absorption Spectroscopy (TDLAS) has a spectral resolution narrower than Doppler line widths, which permits positive identification and unequivocal measurements of components in complex gas mixtures as well as providing an ideal reference standard against which other, less definitive techniques can be compared. Its high resolution makes real-time isotope ratio measurements possible.Instrumentation has been developed to exploit these advantages for the chemical analysis of gas mixtures in various environments . TDLAS systems are now available with a detection limit as low as 25 pptv and a dynamic range of more than 4 orders of magnitude. This permits measurements of species in pristine environments or trace constituents in more polluted air. The rapid time response of these systems has been used to make eddy correlation flux measurements and to get good spatial resolution from fast moving aircraft. TDLAS measurements have been used in the very clean troposphere and in the stratosphere, in polluted urban and rural air, in smog chambers, landfills, industrial sites and indoor work places and in automobile exhausts. Systems have been deployed on mobile vans, ships, aircraft, balloons and rockets.In this paper, the principles and requirements of TDLAS are briefly discussed, and a typical instrument described with some current and future technical developments. Some applications of TDLAS are also presented with the results of recent field measurements.