This paper examines the concepts involved in communication between systems in a general purpose computer network. The idea of a process as a communicating entity is discussed. Facilities for the support of processes which enable inter-process communication are described for a single system. Characteristics of a general purpose network are presented, introducing the network environment and some of the basic communications problems encountered. Requirements for inter-process communication within a network environment are then examined. A hierarchy of communications levels is evolved. The highest level of the hierarchy is that of process/ process communications. Chapter 2 examines the concepts and requirements for implementation of process/process communication in a network. The primitive operations required and their implementation via the network control programs are considered. Chapter 3 examines the interconnection of network control Programs to provide the second level of the hierarchy. This connection is defined by a protocol agreed between the network control programs. General requirements of the protocol are defined and examples of current systems are given. The connection of a host computer system to the communications medium, a network of interconnected nodes, is described in chapter 4. The description is at two levels; the first that of software, the second, the actual hardware connection. This constitutes a host/node protocol. Examples of the general concepts are given from current implementations. At the lowest level, the inter-communication of the nodes is considered, Mechanisms for efficient and error-free transfers of data arc outlined in a feed-back correction protocol. General principles are again illustrated by current systems. As a logical extension of chapter 5, chapter 6 examines line control in a more general network environment. Chapters 7 and 8 outline the basic concepts involved in network topology and routing. These are vast fields for study in their own right and the chapters concerned present only a basic introduction. The requirements for direct terminal handling within a network are considered in chapter 9. The distribution of responsibility between elements of the communications system to provide effective terminal service is examined. Chapter 10 outlines some future developments from present trends. An appendix is included showing an application of process co-operative communication. This is based on the Glasgow University Multum operating system, developed concurrently with this research.