Concussion, from the Latin concutere (“to shake violently”) or concussus (“action of striking together”), is the most common type of traumatic brain injury. The terms mild brain injury, mild traumatic brain injury (MTBI), mild head injury (MHI), minor head trauma, and concussion may be used interchangeably, although the last is often treated as a narrower category. Although the term “concussion” is still used in sports literature as interchangeable with “MHI” or “MTBI”, the general clinical medical literature now uses “MTBI” instead. In this article, “concussion” and “MTBI” are used interchangeably. Frequently defined as a head injury with a temporary loss of brain function, concussion causes a variety of physical, cognitive, and emotional symptoms, which may not be recognized if subtle. Treatment involves monitoring as well as physical and cognitive rest (reduction of such activities as school work, playing video games and text messaging). Symptoms usually resolve within three weeks, though they may persist or complications may occur. Those who have had one concussion seem more susceptible to another, especially if the new injury occurs before symptoms from the previous concussion have completely resolved. There is also a negative progressive process in which smaller impacts cause the same symptom severity. Repeated concussions may increase the risk in later life for dementia, Parkinson’s disease, and/or depression. A variety of signs accompany concussion including somatic (such as headache), cognitive (such as feeling in a fog), emotional (such as emotional changeability), physical signs (such as loss of consciousness or amnesia), behavioral changes (such as irritability), cognitive impairment (such as slowed reaction times), and/or sleep disturbances. A 2010 Pediatrics review article focusing on children and adolescents noted that fewer than 10% of sports-related concussions had associated loss of consciousness. Due to varying definitions and possible underreporting, the rate at which concussion occurs annually is not accurately known, but is estimated to be more than 6 per 1,000 people. Common causes include sports injuries, bicycle accidents, car accidents, and falls, the latter two being the most frequent causes among adults. In addition to a blow to the head, concussion may be caused by acceleration forces without a direct impact, and on the battlefield, MTBI is a potential consequence of nearby explosions. It is not known whether the brain in concussion is structurally damaged or whether there is mainly a loss of function with only physiological changes. Cellular damage has reportedly been found in concussed brains, but it may have been due to artifacts from the studies. It is currently thought that structural and neuropsychiatric factors may both be responsible for the effects of concussion.