Because of its importance in radiative transfer, convection, general circulation, and the hydrological cycle, atmospheric water vapor plays a crucial role in understanding atmospheric processes. For example, since water is the most active infrared molecule in the atmosphere, water vapor response is a major factor in any global warming triggered by increasing carbon dioxide. In addition, atmospheric aerosols also have a significant impact on the earth's climate by scattering and absorbing solar radiation and by altering the physical and radiative properties of clouds. Clouds also play an active role in the atmospheric radiation balance. A Scanning Raman Lidar (SRL) was developed and is used to provide frequent and accurate measurements of water vapor, aerosols and clouds to study these atmospheric processes. For this system, laser scattering by molecules (water vapor and nitrogen) and particles (suspended aerosols and cloud droplets or ice crystals) is detected as a function of altitude. Water vapor mixing ratio, which is the ratio of the mass of water vapor to the mass of dry air, is computed from the ratio of the Raman scattering from water vapor and nitrogen. When combined with measurements of temperature, the lidar water vapor data gives profiles of relative humidity. The lidar water vapor data acquired during field experiments have been used to validate radiative transfer models and study atmospheric features such as fronts, gravity waves, drylines and bores. The water vapor measurements also assess the quality of ground, balloon, and space-based sensors. These water vapor data have been used to determine how advanced statistical techniques (spectral, multifractal, and wavelet analysis) can be used to help understand the nature and causes of atmospheric structure and variability. In addition to measuring water vapor, the Scanning Raman Lidar simultaneously measures both aerosol backscattering, extinction and depolarization. Research is underway to use measurements from the system to quantify cirrus cloud ice water content and warm cloud liquid water content.