This paper describes the procedures and algorithms for the laboratory calibration and field data retrieval of the NASA Langley/Ames Diode Laser Hygrometer as implemented during the NASA Transport and Chemical Evolution Over the Pacific (TRACE-P) mission during February–April 2000. The calibration is based on a National Institute of Standards and Technology traceable dew point hygrometer using relatively high humidity and short optical path length. Two near-infrared water vapor absorption lines of widely different strengths are used to increase the dynamic range of the instrument in the course of a flight. The laboratory results are incorporated into a numerical model of the second harmonic spectrum for each of the two spectral window regions using spectroscopic parameters from the HITRAN database and other sources, allowing water vapor retrieval at upper tropospheric and lower stratospheric temperatures and humidity levels. The data retrieval algorithm is simple, numerically stable, and accurate. A comparison with other water vapor instruments on board the NASA DC-8 and ER-2 aircraft is presented.