The Network Interface Machine (NIM) is a microprocessor-based interface used to connect diverse computers and terminals to a communications network. The purpose of using the NIM, as contrasted to direct connection, is to offload the network interface mechanisms completely from the user devices and the underlying data communication facility. This offloading provides a portable, simplified, and easily implemented, network interface which is adaptable to computers, terminals and networks with diverse characteristics. This thesis defines the architecture, internal design and implementation approach of the NIM, and the high-level description language used to specify the characteristics of a particular NIM implementation. NIM software is produced from the high-level description by program-generation techniques. The thesis surveys data communication concepts, network architectures and implementation techniques. The benefits of general-purpose local computer networks -are discussed, and the requirements that such networks must satisfy are identified. The NIM concept is proposed as an answer to these requirements. The architecture, protocols, implementation and description language for the areas of network control, terminal virtualisation, and external access interfaces are described in detail.