This paper reviews the demand for mobile satellite communications and radiodetermination services from the maritime, aviation, and land mobile communities. It briefly examines existing alternatives and the factors that are affecting the growth and development of mobile satellite communications, with particular reference to INMARSAT. Future mobile earth station standards and services are described as are the key technological developments and challenges, which need to be met before mobile satellite services-on aircraft, the smallest of vessels, and land vehicles-become widely available at significantly lower costs than today. The authors are cautiously optimistic that the 1990\´s will see a rapid growth of mobile satellite services, but such growth will depend in large part on the resolution of certain institutional and technical problems, particularly relating to the limited mobile satellite spectrum available at

-band.