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To ensure the reliability of oil-immersed power transformers in long-term operation, the influence of aging on various characteristics of their insulating elements must be clarified. The present paper reviewed the degradation status of insulating oil and paper of field-aged transformers, covering investigative results ranging from about 30 years ago to a relatively recent period. The field transformers treated were diverse in terms of their manufacturing year, manufacturer, structure, and voltage class. Despite the examination results of such wide-ranging transformers, there were similar changes in various characteristics due to the aging of insulating oil and paper. For example, as for the insulating oil, due to the aging for about 30 years, the characteristics of the total acid value and volume resistivity had degraded, whereas the breakdown voltage decreased only slightly. Regarding the insulating paper, mechanical characteristics such as the average polymerization degree (DP) were subject to age-related decline, whereas the breakdown voltage did not significantly decrease as with insulating oil. Therefore, based on use of about 30 years, since the life of a transformer is determined by factors such as the degradation of mechanical strength due to thermal stress, insulation degradation is considered less likely to pose a problem at an early stage. Even though it is difficult to directly measure the degree of degradation of insulating paper, this can be indirectly estimated by measuring the CO2 and CO content and furfural content in insulating oil. On the other hand, given the increasing number of transformers remaining in service over 30 years, which had been deemed the expected service life, consideration must be taken to aging degradation of the insulation performance. It emerged that, in order to identify the degree of degradation of these transformers, it was effective to manage the volume resistivity and dielectric loss tangent trends in electric- l terms, the interfacial tension trend in physical terms and the total acid value trend in chemical terms. Analysis of the insulating oil components is also considered an effective means of evaluating aging degradation.