The availability of new lasers in the near and mid infrared spectral region has led to the development of sensors for gas measurements that are now applied extensively in the process industries. Based on tunable diode laser absorption spectroscopy (TDLAS) molecules like O2, CH4, H2O, CO, CO2, NH3, HCl and HF can be detected in-situ with high selectivity and sensitivity in continuous, real time operation. Using sensitive detection techniques like wavelength modulation spectroscopy (WMS), often low ppb and ppm concentration measurements with 1s integration time are feasible. Detection limits can be improved by using extractive sampling and a long multi-pass cell. TDLAS has become an accepted technique in the process industries for difficult measurement tasks, because it is compatible with high temperatures, pressures, dust levels and corrosive media. Gas concentrations, temperatures, velocities and pressures can be determined. TDLAS is used widely for continuous emission monitoring and process control with over 1,000 field instruments worldwide. In this article, after an introduction to the basics of TDLAS, several interesting applications and installations in various process industrial units with some examples from other industries are reviewed.