Process industries such as a petrochemical facility, or a plant that produces hydrogen, biogas, fertilizers or pharmaceuticals require the use of products that are hazardous for both humans and the environment. Multiple measures are available to the process safety engineer to control these risks. Among these, functional safety contributes to the prevention of accidents. The latest version of the IEC 61511 standard (Functional safety - Safety instrumented systems for the process industry sector), published in 2016, proposes an approach to implementing functional safety and ensuring its effectiveness. This Unit introduces the concepts found in the standard, and explains its benefits and limitations in order to better understand and use it.