The development of satellite communications over the past two decades has been very rapid. At the same time, space and electronics technologies have progressed sufficiently to allow satellite systems to keep pace with user requirements and to expand in several dimensions, including capacity, coverage, performance, reliability, and variety of services. The principal services provided by satellite systems have been long-distance trunks for telephone circuits and television program distribution. Recently, data transmission and network services have been added to meet emerging requirements for "office of the future" and other business applications. Data services are now expected to be the most rapidly expanding element of satellite communications.