The composition, winds, and temperatures of Earth’s mesosphere and ionosphere/thermosphere are determined by the complex interplay of chemical, dynamical, and thermodynamic processes driven by forcing from both above and below. The forcing of these regions and the many different processes involved vary over a wide range of spatial and temporal scales, hindering accurate determination of these fundamental parameters critical to satellite operations and radio communications. Past and current space missions, field campaigns, as well as theoretical and modeling advances, have led to a new understanding of how and why the dominant drivers of upper atmospheric composition, winds, and temperatures in these regions change over different spatial and temporal scales. This session, therefore, seeks to highlight various topics surrounding middle and upper atmospheric composition, wind, and temperature measurements, their variability, and the prominent mechanisms that drive their spatial, seasonal, and inter-annual variability from theoretical, observational, and modeling perspectives.