Reliability evaluation we have discussed before has been focusing on methods of using event data analysis, mainly by using failure data to find out the reliability level. This method is essentially analyzing reliability from management perspective and statistic perspective, while less from the engineering perspective. Data from fields contain noise, outlier, inaccuracy. The resultant evaluation accuracy is thus compromised. With the growing concern of reliability issue for the modern equipment’s such as cellphone, smart home, autonomous drone, the reliability evaluation has gone beyond the original domain. In the industries, reliability evaluation from field data might be too rough in its accuracy or infeasible as it often lacks data. There is another domain of reliability analysis which analyzes the probabilistic behavior of the system, where the reliability level is evaluated from a probabilistic analysis, for example, evaluating reliability from thermal models where some parameters and measurements are uncertain for electronic units; evaluating the probability of bridge collapsing by mechanical analysis where operating condition is uncertain in structural engineering. This kind of analysis is also named probabilistic design. This Chapter addresses this method. We start from the simplest stress-strength analysis then to the more general limit-state analysis.