The university of Illinois (Urbana) lidar system has been developed to study the atmospheric sodium layer near 90 km altitude through the mechanism of resonance scattering. The photocount data are processed using digital smoothing filters to obtain continuous estimates of the sodium density versus altitude. The filter cutoff frequency is related to the height resolution and accuracy of the estimated profile. Lidar photocount data processed using this filtering technique show wave-like structures in the sodium layer which move downward with time. The waves have typical wavelengths of 3-15 km and phase velocities of less than 1 m s-1. The movement of these structures seems to be independent of the motion of the bottomside of the layer, which also has been observed to move up or down by as much as 2 km over a period of a few hours. The presence of these motions is attributed to gravity waves and tidal oscillations present in the layer.