Practice guidelines universally recommend an initial period of rest for people who sustain a sports-related concussion or mild traumatic brain injury (MTBI) in daily life or military service. This practice is difficult to reconcile with the compelling evidence that other health conditions can be worsened by inactivity and improved by early mobilization and exercise. We review the scientific basis for the recommendation to rest after MTBI, the challenges and potential unintended negative consequences of implementing it, and how patient management could be improved by refining it. The best available evidence suggests that complete rest exceeding 3 days is probably not helpful, gradual resumption of preinjury activities should begin as soon as tolerated (with the exception of activities that have a high MTBI exposure risk), and supervised exercise may benefit patients with persistent symptoms.