The paper summarizes investigations on telephone interference carried out in this country between 1934 and 1944. In many cases, experimental work could not be pursued to a logical conclusion owing to the war, and has therefore served only to indicate the need for future research. The paper is divided into six sections:Electromagnetic Induction at Fundamental Frequency.—A comparison of calculated and measured values of induced voltage at various sites shows that in most cases good agreement is possible with the available data on earth resistivity. Further work is required on the screening effect of various earthed conductors. The precautionary measures available for power and telephone systems to avoid damage to plant and injury to personnel from high induced voltages are discussed.Interference at Audio Frequencies.—Results of tests are given which show that noise interference is serious from faulty power lines which are maintained in operation through the use of arc-suppression coils. The propagation of harmonic currents on long power lines requires further study; the results so far obtained indicate that an analysis of neutral currents is insufficient to forecast the noise interference possible. Attention is drawn to the possibility of interference from a power line supplying a large rectifier.Multiple Earthing of High-Voltage Systems.—Tests which have enabled conditions to be laid down for authorizing the multiple earthing of h.v. systems are reviewed.Multiple Earthing of Low-Voltage Systems.—Tests show that the interconnection of l.v. systems, each earthed at one point, is unlikely to cause interference.Apparatus Developments.—Gas discharge tubes, noise-eliminating filters, and an improved psophometer are described.Rise of Earth Potential.—Records of damage sustained and the precautionary measures which can be taken are discussed.