RESUME / ABSTRACT The trend to higher lids of weather forecast models continues with the recent rise of ECMWF's model top to 0.01 hPa (80 km), highlighting the need for an improved understanding of vertical coupling in data assimilation systems. This trend is fuelled by the desire to better assimilate nadir satellite soundings which sense deep layers of the atmosphere. Because tropospheric channels also sense the stratosphere, an improved representation of the middle atmosphere can improve weather forecasts. However, vertical coupling of the various regions can occur through both the analysis and forecast steps of the assimilation cycle. Because waves propagate upward, they can carry information from the model troposphere upward through background error covariances, resolved waves and parameterized gravity waves. A nice example of vertical coupling is seen in the phenomenon of stratospheric sudden warming, a not uncommon occurrence in Arctic winters. Understanding and predicting such polar phenomena will be a focus of stratospheric projects for the coming International Polar Year (2007-8).