The Safety Critical Systems Handbook: A Straightforward Guide to Functional Safety, 6th edition is a practical, fully updated guide to achieving safer technology in systems that protect workers and the public against injury and death, while also addressing environmental risk. Building on IEC 61508 and related guidance, this sixth edition incorporates the latest 2026 updates, including clarified scope for artificial intelligence (software technology class), expanded coverage of human factors and security, and strengthened requirements for competence and levels of independence in functional safety assurance. It introduces enhanced technical expectations including relocated semiconductor content, new requirements for diagnostic functions and common cause failures, and expanded guidance on software off-line support tools. For systems engineering, it incorporates and extends IEC TS 61508:2016 to support Systematic Capability SC3 and SC4, clarifies the difference between SIL (required) and SC (achieved), and strengthens software lifecycle management, traceability, and regression testing, including new guidance for data-driven systems.