The paper highlights the differences in failure modes and electrical and mechanical strengths of porcelain and toughened glass insulators that have been in service for many years. The findings are based on a vast body of published literature, as well as a study performed on insulators that were in service for up to 70 years. The insulators were tested for insulation resistance, dielectric strength, porosity and residual mechanical strength. The microstructure of the porcelain was analyzed using a scanning electron microscope. Significant differences in the compositional and mi-crostructure details of porcelain insulators were observed. The electrical and mechanical strengths of porcelain insulators was found to vary significantly from one manufacturer to another, with some aged insulators showing marginal changes and others showing significant reduction in these parameters. Glass insulators evaluated showed small reduction in their residual mechanical strength when compared to new ones. These differences should have significant impact on system reliability and maintenance procedures adopted by utilities.