Atmospheric visibility has been measured with the lidar technique using the “slope method.” The system is briefly described and some aspects of slant path visibility measurements for aeronautical applications are discussed. Measurements in dense fog show the existence of a structure with layers of different attenuations. Transmissometer and lidar visibilities are compared along horizontal and slant paths. Visibilities obtained with the lidar technique in fog situations show good agreement with the simultaneous transmissometer data. However, it must be noted that lidar measurements give values greater than the transmissometer data; the systematic discrepancy is ∼30 m for slant visibilities below 300 m, and cannot be explained by multiple scattering effects alone.