The mesosphere and lower thermosphere (MLT) is that part of the atmosphere at heights of ~ 50 / 110 km. This project used an array of sophisticated meteor radars, the international EISCAT radar in Scandinavia, the NASA Aeronomy of Ice in the Mesosphere satellite and three numerical models to study the winds, tides and waves of the MLT and to investigate how they control Polar Mesospheric Clouds. A particular focus of the work was to understand how solar variability influences the atmosphere at these heights and to study how coupling processes connect the MLT to the underlying lower atmosphere and the upper atmosphere above.