The paper first outlines the inherent strengths and weaknesses of mobile and fixed terminals in HF communication systems. Conventional HF system design and operating procedures are then described. The improvement in performance achievable by the use of various forms of diversity combining is quantified, and it is shown how a number of fixed HF terminals can be combined into a geographical diversity network, which can be accessed by mobile terminals. Specific problems of interconnection, data transmission and system control are considered in greater depth.