Spaceborne lidars will give a new view of the vertical distribution of atmospheric aerosols and clouds. CALIPSO will be launched in the fall of 2004 and will provide, for the first time, a global picture of the profile of atmospheric scattering layers using an onboard lidar radiated at 0.532 and 1.064 ¿m. CALIPSO will fly in an orbital formation with passive radiometers, such as the Moderate resolution Imaging Spectrometer (MODIS) and the Polarization and Directionality of Earth's Reflectance (POLDER) instruments, that monitor Earth's atmosphere. The purpose of this paper is to analyze the improvement in retrieval capabilities of profiles of aerosol optical properties using a synergy between passive and active (lidar) remote sensing techniques. Aerosol properties derived from the MODIS spectroradiometer are used to constrain the inversion of the lidar signal in terms of aerosol optical thickness and effective radius. We use the lidar spectral backscattering coefficient between 0.532 and 1.064 ¿m to determine the profile of backscatter-to-extinction ratio. The effective radius of an assumed bimodal aerosol size distribution is then retrieved as a function of the altitude. A sensitivity analysis demonstrates the robustness of the inversion procedure in case of noise detection and calibration error. The algorithm has been tested during the Saharan dust experiment, which took place in the northeastern tropical Atlantic in September 2000. The vertical profile of extinction compares well with in situ measurements of the aerosol extinction. Profiles derived from lidar measurements on 25 September highlight the presence of the Saharan air layer located between 2.2 and 4.5 km with particle effective radii of 1.19 ¿ 0.6 ¿m. Another dust layer within the sub-Saharan transition layer over the marine boundary layer is also observed, with particle radii significantly smaller than within the Saharan air layer.