Defined as "the ability of a system to perform when and as required" (IEC), Dependability is also the science of ensuring this ability and acquiring justified confidence in it. Linked to risk management and often summarised under the term RAMS (Reliability, Availability, Maintainability, Safety), it has become essential in industrial fields (energy, transport, etc.) where failures can have major consequences for people, the environment, the economy and, more broadly, society. To provide scientific arguments for the design of new systems, we invite you to enter the world of operational safety: "know what is" (feedback) to "predict what will be" (methods and tools for analysis and calculation developed in this module) in order to "improve it as much as possible" (decision support to choose the best compromises).