The use of statistical approach for reliable and optimal design of systems, including electronic systems, has become quite popular. Statistically designed experiments/simulations have been used extensively for estimating or demonstrating existing reliability by identifying the important parameters (factors) affecting reliability out of many potentially important ones. This statistical approach ensures the performance and safety of critical electronic systems employed for space, nuclear, automotive, defence, sub-marine and aerospace applications, to name a few. A statistical life model becomes more reliable, valuable, and useful, if it fits the experimental data quite well. In this section, we begin with the definition of the basic concept of reliability and lifetime estimation concepts. The reliability of a system is defined as the probability that it will perform its required function under stated conditions for a stated period of time.