The subject of spacecraft fault detection, isolation, and recovery, also known as fault protection, is introduced along with standard terminology. A general description of a typical fault protection system is presented with a specific emphasis on the application to guidance, navigation, and control (GN&C) subsystems. The functions of a fault protection system for a notional three‐axis controlled GN&C subsystem are presented as examples. Finally, the role of various types of redundancy and the desire for graceful degradation in the presence of failures in fault protection systems are discussed.