This overview of fatigue includes fatigue definitions, the measurement / assessment of fatigue, and the performance, mood, and safety problems associated with fatigue in the operational setting. The physiological bases of fatigue are discussed, so the reader understands that fatigue is a physiological phenomenon that is not "just a state of mind". Scientifically-valid countermeasures are discussed and data from a variety of sources are included to provide readers with a "toolbox" from which they can choose solutions to fatigue-related problems. The book is of interest to aviation crews in both civilian and military sectors, managers as well as aviators, flight deck as well as maintenance crews. It aims to be 'user-friendly', although scientific information is included to help the reader understand why certain behaviours occur.