The paper presents a survey of those principles in aerial design which have come to the fore during-the war in the development of aerials for radar equipments. It is intended to serve as a co-ordinating introduction to specialist papers which willfollow. It is assumed that the reader is familiar with pre-war theory and practice in relation to aerials.On metre wavelengths the aerials are divided into those working on the “flood-lighting” and those working on the “beam” principle. The beaming in the vertical plane is seldom sufficiently sharp to avoid effects from ground reflections, which therefore play a predominant role in methods of elevation-finding.In radar direction-finding from aircraft with metre waves, use is made of overlapping “polar diagrams” provided either by self-directive aerials, or by aerials so positioned that the metal body of the aircraft causes them to have the required directive properties.With centimetre waves it is possible to produce beams so sharp that complications due to reflections from the ground can, to a large extent, be avoided. It is explained how the principles of optical diffraction theory are applied to the design of linear arrays fed from wave guides, so as to produce “fan-beams” of radiation narrow in one plane and sharp in the perpendicular one. The production of “pencil-beams,” sharp in two planes at right angles, is also discussed.Some account is given of new developments in the use of slots as aerials, in the design of wide-band aerials, and in the theory and design of end-fire arrays of the Yagi type.