A perfect process would have no hazards, but perfection is impossible in the real world. Nearly all process units have inherent risk associated with their with a risk reduction strategy relying on a wide variety of safety systems. This article focuses on the most common safety systems for managing process deviations during planned operating modes-instrumented safety systems (ISSs), such as safety alarms, safety controls, and safety instrumented systems. Rigorous quality assurance is necessary to achieve real-world risk reduction, so this article follows the Plan. Do, Check, and Act process to discuss quality assurance and its application to ISS. (C) 2008 American Institute of Chemical Engineers Process Saf Prog 27: 323-327, 2008