The transition between the middle atmosphere and the thermosphere is known as the MLT region (for mesosphere and lower thermosphere). This area has some character- istics that set it apart from other regions of the atmosphere. Most notably, it is the altitude region with the coldest overall temperature and has the unique characteristic that the temper- ature is much colder in summer than in winter. The summer to winter temperature gradient is the result of adiabatic cooling and warming associated with a vigorous circulation driven primarily by gravity waves. Tides and planetary waves also contribute to the circulation and to the large dynamical variability in the MLT. The past decade has seen much progress in describing and understanding the dynam- ics of the MLT and the interactions of dynamics with chemistry and radiation. This review describes recent observations and numerical modeling as they relate to understanding the dynamical processes that control the MLT and its variability. Results from the Whole Atmo- sphere Community Climate Model (WACCM), which is a comprehensive high-top general circulation model with interactive chemistry, are used to illustrate the dynamical processes. Selected observations from the Sounding the Atmosphere with Broadband Emission Ra- diometry (SABER) instrument are shown for comparison. WACCM simulations of MLT dynamics have some differences with observations. These differences and other questions and discrepancies described in recent papers point to a number of ongoing uncertainties about the MLT dynamical system.
