In 1968 the Advanced Research Projects Agency (ARPA) of the U.S. Department of Defense began implementation of a computer­ communication network which permits the interconnection of heter­ ogeneous computers at geographically distributed centres through­ out the United States. This network has come to be known as the ARPANET and has grown from the initial four node configuration in 1969 to almost forty nodes (including satellite nodes in Hawaii, Norway, and London) in late 1973. The major goal of ARPANET is to achieve resource sharing among the network users. The resources to be shared include not only programs, but also unique facilities such as the powerful ILLIAC IV computer and large global weather data bases that are economically feasible when widely shared. The ARPANEr employs a distributed store-and-forward packet­ switching approach that is much better suited for computer­ communications networks than the more conventional circuit-switch­ ing approach. Reasons favouring packet switching include lower cost, higher capacity, greater reliability and minimal delay. All of these factors are discussed in these Proceedings.