Transmission and distribution networks of most utilities worldwide have a substantial quantity of porcelain and toughened glass insulators that have been in service for over 30 years; a number that has been traditionally considered as useful life for planning purposes. Many utilities are implementing programs to replace older or defective insulators for improving reliability and also for upgrading their lines. In order to help prioritize line maintenance it is important to identify the condition of insulators and replace those insulators whose performance has reduced significantly from their new condition, hence threatening reliability. In this project, porcelain and toughened glass insulators that were in service for periods ranging from 10-30 years were removed from several locations. Electrical (puncture strength, flashover voltage) and mechanical (tensile strength) tests were performed to assess any reduction in performance. Diagnostic tests such as analysis of bonding agents, identification of internal failure paths were performed to understand failure causes. A sufficiently large number of samples were evaluated and this permitted condition assessment with statistically significant confidence limits.