Decision making related to the safety of complex technologies is difficult in the best of circumstances. In the face of significant uncertainty, decision makers rely on input from a variety of sources, including the results of system safety analyses. System safety is a widely accepted management and engineering approach to identify, analyze, and address risks in complex systems such as chemical processing plants, nuclear power plants, railroads, airplanes, and rockets. When used correctly, system safety methods can provide tremendous benefits, focusing resources to reduce risk and improve safety. But for a variety of reasons system safety analyses may fail to identify hazards, assess risks, implement safeguards properly, or verify that risks have been reduced. A decision maker must be able to differentiate between effective and poor system safety efforts in order to make critical safety decisions. One of the best tools available to a safety decision maker is asking intelligent questions to try to understand whether the system safety approach used truly reduces risks. Essential Questions in System Safety provides probing questions that should be asked by any organization building and operating complex systems. These questions should serve as a springboard to additional inquiries and evaluations by safety decision makers. The questions provided here may be used with the companion book The System Safety Skeptic: Lessons Learned in Safety Management and Engineering to help improve the safety of complex processes and systems.