Tests on hydrocarbon oils for cable systems are reviewed on the basis of their ability to estimate the effect of an oil on six insulating properties: electrical strength, water sensitivity, thermal resistivity, bendability, loss factor, and susceptibility to corona in cavities; the sensitivity of an oil to oxidation, corona, and mixing with other oils; the fire, explosion, and personnel hazards involved in the use of an oil; and the continuity of oil production. To estimate the effect of a homogeneous hydrocarbon oil on insulation properties, viscosity and some measure of conductivity, preferably dissipation factor, are usually adequate. The desired level of viscosity may be either high or low depending on the cable system in which the oil is to be used. Conductivity should be as low as compatible with other desired properties. For some cable systems, gas content of an oil should also be low. The water content of clear cable oil is normally so low that its exact level is unimportant. Electrical strength measurements are useful mainly to establish the homogeneity of an oil sample. Sensitivity of an oil to oxidation or to mixing with other oils is best judged by conductivity measurements after a mild oxidation or after mixing with other oils. On the other hand, sensitivity to corona frequently can be adequately estimated by infrared or carbon-type analyses without a simulated corona test. Hazards in the use of an oil and the continuity of oil production can be estimated by measurements of flash point, explosibility limits, effects on animals, specific gravity, refractive index, and absorbance of ultraviolet and infrared energy.