Because of the great increase of speed of modern aircraft, there has arisen a need for the elimination of the drag of external aerials. A common form of the suppressed type of aerial is the slot cut in the metallic skin of the aircraft, the slot behaving in a manner which is complementary to that of the conventional dipole. Measurements of the polar diagrams of slots installed on both full-scale and model aircraft show that the types of polar diagram frequently required on aircraft can be obtained by use of slot aerials, for which there is some agreement between the measured polar diagrams and those estimated theoretically using ray and diffraction theories. Exact diagrams apparently have to be determined experimentally, because aircraft normally do not have simple shapes which are amenable to calculation. The installation of slots has provided no great difficulty at metre or shorter wavelengths, but their application at longer wavelengths will depend upon what can be achieved by matching devices, and upon the limit to which the aircraft structure can be suitably modified.