Reliability theory has certainly played an important role in industrial and system engineering. Customers and users are always looking for high quality and long life products or systems. Therefor, continuous improvement of prod- ucts or systems becomes essential and even critical. The term " reliability " can be de.ned as the ability of a product or system to perform over a period of time according to design speci.cations or according to user expectations. Longer life is identi.ed with greater reliability. In measuring reliability, we are observing lifetimes of products or systems of interest. In order to gain a sound knowledge about product or system failure-time distributions, life testing and reliability experiments are carried out before ( and while) products are put on the market or system are used. Life-testing experimentation is a practical way to determine the product relia- bility and product quality. A life-testing experiment is one in which a sample of the item of interest are subjected to stresses and environmental conditions that epitomize the intended operating conditions.