The principles of intracavity laser spectroscopy (US) together with the experimental conditions which permit the detection of absorption with extremely high sensitivity are briefly described. Special attention is given to the optical parameters which need to be controlled in the U.S laser and to the observational conditions under which quantitative measurements can be made. Applications of ILS to the in situ monitoring of chemical vapor deposition of thin
film materials and the quantitative characterization of absorption lines (e.g., line strengths and collisional broadening coefficients) are described. Recent results on the design and performance of solid-state ILS lasers operating in the infrared also are presented.