One of the cellulose insulation in high voltage power transformers is paper, and it is widely used in transformers due to low cost and excellent physical/electrical properties. However, the performance of paper could be affected by ageing. Ageing of paper is a complex phenomenon and can be influenced by many factors. This PhD thesis aimed to examine the ageing of paper in transformers based on Tensile Index (TI), dielectric strength and Low Molecular Weight Acid (LMA) measurements.The effect of ageing on the dielectric strength of paper was examined, through studying the influence of paper and oil ageing by-products such as LMA and HMA on the AC breakdown voltage. Meanwhile, the end of life for transmission transformer population in the UK was examined by studying the profile of LMA and TI of paper in scrapped transformers, with phase, winding, and layer locations taken into consideration. The relationship between TI, LMA and transformer age was modelled using previously published formulae in literature. The partitioning of LMA and HMA between mineral oil and paper was also investigated. Since TI is not directly measureable when a transformer is in-service, a TI prediction model was developed based on LMA measured in oil.The results indicated that there is no reduction effect of ageing (moderate towards severe) on the dielectric strength of paper, whereas the dielectric strength of paper is mainly influenced by the oil condition. LMA significantly reduces the breakdown voltage and accelerates the ageing of paper in mineral oil. The end of life based on TI is best represented by the 20 % retention of TI. Based on the case studies, the estimated end of life of transmission transformer population in the UK is around 43 years. The vertical distribution profile of LMA in paper and TI of paper along a winding is mainly influenced by vertical temperature profile. As paper ages, the amount of LMA in paper tends to increase as TI reduces. The percentage of LMA in service aged oil is about one fourth of Total Acid Number (TAN). The partition of LMA in mineral oil and paper favours its stay in paper and is heavily influenced by the weight ratio of paper and oil.The performance of the natural ester under high temperature ageing in open or sealed condition was evaluated based on TAN and viscosity measurements. The ratio of LMA and HMA in aged natural ester was examined. The effect of LMA and HMA on the AC breakdown voltage of natural ester and ageing of paper was also investigated. It was suggested that natural ester is suitable for application in hermetically sealed transformers. High temperature oxidation has significant effect on the physical and chemical properties of natural ester. Due to the polar nature of natural ester, the stay of LMA in natural ester is favoured over in paper which could be the reason why the breakdown voltage of natural ester and the ageing of paper are not significantly affected by LMA as in mineral oil.