The significance of gaseous discharges in the electrical breakdown of solid and composite solid-liquid dielectrics has emerged with increasing clarity over the past three decades. Arising from researches in England and North America, substantial advances in the detection and interpretation of discharge phenomena were made in the second decade, principally by the British Electrical and Allied Industries Research Association. Towards the end of the second decade the usefulness of applying discharge tests continuously to extruded insulating coverings over conductors to be operated at high voltages came to be recognized in several quarters in this country, on the Continent and in the United States.Discharges may be detected by measuring the transient electrical disturbance to the system caused by the discharge, or by measuring the electromagnetic r.f. or light waves or the sound pressure waves emanating from the discharge. Reference is made to continuous discharge detectors based on some of these methods.The various practical forms of the method based on electrical observations are discussed in greater detail, since this method is the one more widely used and more highly developed at the present time. The particular arrangement used by the company with which the author was associated is described, and the results obtained in testing more than 400 miles of polythene-insulated high-voltage core are reviewed. Reference is made to technical difficulties encountered in the design and operation of the equipment.The limitations in the usefulness of the present form of testing are stated in the light of the experience gained and some suggestions are made for the further development of the method.