Exponential distribution is the most used distribution in reliability engineering. It is widely applied to model life data for non-repairable systems or the time between failures for repairable systems. Two forms of Exponential distribution can have: the one-parameter Exponential and the two-parameter Exponential. In the one-parameter Exponential distribution, the single parameter represents the failure rate, which characterizes the system’s reliability. The simplicity of having just one parameter makes parameter estimation straightforward. Unless otherwise specified, this chapter refers to Exponential distribution as the one-parameter one.