This document discusses fatigue in the context of military and non-military 24/7 operations. It defines different types of fatigue such as circadian, acute, cumulative, and chronic fatigue. It describes shift work sleep disorder and discusses the fragility of sleep. It also summarizes a simulation model called SAFTE that models fatigue effects and discusses fatigue countermeasures such as sleep hygiene, napping, caffeine use, and stimulant medications.