The polar cap is an important part of the high latitude atmosphere not only because of circulation and horizontal transport in the neutral atmosphere and convection in the ionosphere, but also because of its unique energy sources and sinks. In addition, solar wind plasma is led into the upper atmosphere by the geomagnetic field at the poles, and the polar cap is, as stated by Dr. Roederer in this volume, the place where outer space meets the planet earth. The atmosphere at lower latitudes is well-known to the ground based observer, and the advent of satellite observations was simply the beginning of a new perspective. The exploration of the atmosphere at polar latitudes, however, proceeded in quite the opposite manner, and satellite maps of the polar caps may be compared with a relatively meagre set of ground-based data. Recent efforts to extend the polar observations from the ground have resulted in the need for a review of the physical principles and processes occurring in the polar