A Key Life Test is an accelerated test designed to detect a major component failure mode. The Key Life Test process will be outlined, as applied to predicting the life and reliability of an automotive synchronous timing belt. The main objective of this type of testing is the development of a model to improve quality by designing reliability into the product. A math model will be described to assess severe customer usage, determine the test conditions and acceleration factor and is used to find the timing belt life and reliability. The application of Probabilistic Methods will be explained to predict the belt life and reliability, after the model has been correlated against the test results obtained. It will be shown that the design can be optimized to improve the timing belt reliability using the model.