The water vapor mixing ratio distribution in the lower and middle troposphere has been continuously monitored, using an active lidar system. The methodology of the differential absorption laser method used for these measurements is summarized and related to the corresponding achievements of the experimental system set up at the Haute-Provence Observatory, France. The experimental results emphasize the unique aspect of this remote sensing system for humidity profiling, in terms of continuous monitoring with high temporal and spatial resolutions. Applications to local-scale studies in the boundary layer (simultaneous measurements of humidity and aerosol content, humidity field below a cloud cover) and to mesoscale evolution in the middle troposphere (dry episodes related to troposphere-stratosphere exchanges, total burden measurements) are presented.