This paper describes a lidar system being developed for range-resolved water vapor measurements from an aircraft using the differential absorption lidar (DIAL) technique. The system uses two frequency-doubled Nd:YAG lasers to pump two independently tunable, high conversion efficiency dye lasers which operate in the near infrared between 710 and 960 nm. The "on" and "off" wavelengths which are used in the DIAL measurements are generated in sequential laser pulses with less than 100 ,us separation. The close spacing minimizes concentration errors that might result from changes in atmospheric scattering characteristics during the airborne DIAL measurement. The dye lasers have a linewidth less than 1.0 pm and an operating wavelength which is servo-controlled to less than 0.2 um using a high resolution, wave-length calibration unit. The backscattered lidar returns at the on and off wave-lengths are received by a 35 cm diameter telescope, sequentially detected by a photomultiplier tube, digitized, and stored on high speed magnetic tape. Water vapor concentration profiles are calculated for each measurement in real time by a minicomputer to permit optimum control of the experiment. A detailed description of the airborne DIAL system is given in this paper, and simulations of water vapor DIAL measurements are discussed. Experiment simulations include proposed investigations of water vapor in the boundary layer, the middle and upper troposphere, and the tropopause region. Simula-tions are also presented of the measurement of global water vapor vertical profiles with a Shuttle-borne lidar system which has operating characteristics similar to the airborne DIAL system.