Reliability testing is a crucial process in software development that evaluates a system's likelihood of failure and helps ensure a product meets acceptable reliability standards before launch. Originating from mechanical engineering, reliability testing involves statistical analysis to predict and improve software performance under real-world conditions, with techniques evolving to address modern complexities like distributed systems. Key methods include feature, load, and regression testing, and more recent practices such as Chaos Engineering, which introduces controlled failures to assess system robustness. The testing process relies on metrics like Mean Time Between Failure (MTBF) and models such as prediction, estimation, and actual models to track and enhance reliability throughout the development lifecycle. The practice is integral for planning, budgeting, and minimizing post-launch issues, with roles spanning across QA, SRE teams, and now developers, as part of the shift-left movement. Ultimately, reliability testing aims to deliver more robust software, reduce customer churn, and improve overall user experience.