COST (cooperation in the field of scientific and technical research) is a framework for scientific and technical cooperation which facilitates the coordination of national research on a European level, and is concerned with basic and precompetitive studies as well as activities of public utility. Earth-satellite radiowave communications are central to many new radio services which are designed to meet increasing communications requirements. The proper understanding of the influence of the upper atmosphere on radiowave propagation is therefore essential to support the design, implementation and operation of many modem communication systems. The main objective of the COST 271 Action, entitled "Effects of the Upper Atmosphere on Terrestrial and Earth-Space Communications" (EACOS), is to promote, stimulate and coordinate European research in ionospheric and plasmaspheric areas. The work of the Action under four working groups is described and current results presented.