During service, overhead transmission line insulators lose their functionality due to electrical stresses and environmental conditions causing incessant flow of leakage current (LC) on their surface thereby leading to insulator ageing. Ceramic glass insulators are the oldest and the most used type of insulator, some operating for decades and approaching the end of their projected life span. A technique to assess as well as diagnose ageing glass insulators has yet to be proposed. This book, therefore reports a new method of assessing the surface condition of ageing transmission line glass insulators which is cheaper, more efficient and less time-consuming as compared to the traditional methods currently practiced. Insulator surface leakage current (LC) was comprehensively investigated and a strong correlation was observed between the LC frequency components and the surface condition of the ageing insulators. This finding suggests the viability of online monitoring of aged transmission line insulators. This investigation should be useful to the electric utility industry and power engineers.