The paper presents briefly the principles underlying the current development of a high-frequency directional aerial system which can be steered in both azimuth and elevation. The system comprises a number of omnidirectional unit aerials whose outputs (in the case of reception) may be brought approximately into phase with each other for any desired combination of frequency and direction by means of direction-control equipment which is basically a high-speed computer. The aerial system provides a number of independent outlets, each of which may be individually steered by the direction-control equipment. An experimental system based on these principles and designed for the reception of high-frequency signals from transmitters at long distances will be described in later papers.The present paper mentions briefly some of the potential advantages of this type of aerial system, e.g. flexibility of design and operation, and notes that the basic principles are applicable to both transmission and reception.