The radial lip shaft seal is a necessary, important component of modern machinery. A multitude of materials and designs exist for a variety of applications. The manufacturer must choose a seal type that will satisfy the conditions of the application with high reliability and long life. Confidence in the choice is gained through functional testing. Typical functional tests involve four to twelve seals. These tests are usually run under accelerated conditions. The seals must perform without leakage for a given period of time (typically 100-500 h). This technique (success testing) provides limited information with low confidence levels. Nothing is learned about the failure rate distribution of the general population of seals.A more meaningful statistical procedure, Weibull analysis, is discussed and applied to seal testing. Experimental data are supplied and analyzed. The Weibull charts are used to determine the reliability and the limitations of seal design and material with high confidence levels.