The dynamical and thermodynamical effects of tides are the dominant cause of large scale variability in the mesosphere and lower thermosphere. Recent realizations that both migrating and non-migrating components attain significant amplitudes in this region imply that the large scale structures have considerable complexity. In this paper, the nature of this complexity is summarized and its dependence on the source amplitudes and filtering lower in the atmosphere are reviewed. The dependence of the thermal and vertical motions associated with this variability are presented and their consequences for airglow and chemical heating explored. Of some interest is determining whether non-linear interactions between tidal components is important. Some analysis of this question using the extended CMAM is presented which suggests these interactions may be more difficult to identify than previosly thought. Directions for future research include the identification of the appropriate tidal sources, determining the dissipation mechanisms and determining the global consequences of the tidal stirring of the atmosphere in the MLT region. Strategies for answering these questions are summarized and the role of global campaigns such as those proposed for CAWSES II in providing answers is noted.