An active topic of current research in aeronomy is the study of the dynamics of the mesosphere and lower thermosphere (MLT) from 60 to 130 km, especially in regard to the influences that govern variability. The physical processes of this region are diverse and complex with strong coupling between the MLT and the adjacent atmospheric regions brought about largely by the propagation and dissipation of atmospheric gravity waves (GWs) from sources above and below. The measurements of MLT winds and temperatures required for such studies represent daunting technical challenges. At low and midlatitudes the mesosphere inversion layer (MIL) phenomenon, a ~10km wide region of enhanced temperatures (ΔT~15--50 K), is observed with great regularity in both the upper mesosphere (60--70 km) and the mesopause (90--100 km). Observations are largely based upon Rayleigh and Na temperature lidar systems but coherent radar observations have shown that the MIL phenomenon is linked to layers of turbulence occurring in both the topside and the bottomside regions. GW activity is believed to play an important role in the development of a linkage between the MIL and the tidal structure through GW coupling that results in an amplification of the tidal thermal structure. This linkage is readily evident for the upper MIL but is seen only occasionally for the lower MIL. Further study of MIL properties should emphasize continual 24 hour temperature observations, especially for the lower MIL region, to confirm the linkage of the development of the MIL to the MLT tidal structure. ¿ 2000 American Geophysical Union