Insulators and external insulation on transmission lines and substations are exposed to numerous electrical, mechanical, and environmental stresses that may contribute to reduced performance and life expectancy. Through monitoring the performance of in-service insulators and researching the abilities of insulators, much is learned about how to use them effectively and identify the need to replace them. EPRI is developing the knowledge and tools to help utilities select, specify, apply, inspect, assess, and maintain these components to improve performance and extend service life.