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A short review of the constructive solutions generally adopted in re cent years in manufacturing high voltage current transformers is given, and the problems involved in designing the insulation of such equipment are discussed.The main causes that can reduce the life of the high voltage current trans former insulation are examined in the paper, as well as the problem concerning the most suitable and meaningful quantities to be measured on current transformers in service.On the basis of authors' experience a correlation between measured values and aging conditions of the insulation is made, and upper limit values are also suggested,in order to establish if the tested transformer can remain in service.Some considerations about the actual possibilities of reconditioning aged trans formers are finally made, and a proposal for revising the dielectric tests up to now standardized for new current transformers is reported in appendix.