Fatigue is one of the most common complaints of concussive brain injury patients. Readers will justifiably hope for a more scientifically satisfying label for what, at first glance, might seem a rather ephemeral concept. The authors of this chapter have taken up that definitional challenge, and much more. Their narrative encompasses the epidemiology, measurement, risk and causal factors -- from demographics (e.g., sex) to gene variation -- and neuroanatomical correlates of post-concussive fatigue. Although the concept of a unitary post-concussive syndrome is insupportable, the authors report that fatigue is very often associated with other commonplace sequelae such as perceived stress, somatic symptoms, anxiety, depression, sleep disorders, and endocrine dysfunction. The consequences transcend malaise and inefficiency, since fatigue is linked to sadness and perceived (and perhaps objective) cognitive disability. As the authors explain, a vicious cycle may emerge in which the causes and consequences of fatigue are mutually reinforcing. Perhaps of greatest practical value, the chapter concludes with a detailed, sophisticated, multi-pronged management algorithm that offers clinicians a robust guide to reducing the harms of post-concussive fatigue.