Concussion, which comes from the Latin concutere meaning “to shake violently”, is the most common and least serious type of traumatic brain injury. The term ‘concussion,’ has been used for centuries and is still commonly used in sports medicine. Frequently defined as a head injury with a transient loss of brain function, concussion can cause a variety of physical, cognitive, and emotional symptoms. Concussion can be diagnosed and assigned a level of severity based largely on symptoms. Treatment involves monitoring and rest. Symptoms usually go away entirely within three weeks, though they may persist, or complications may occur. Repeated concussions can cause cumulative brain damage such as dementia pugilistica (boxer’s dementia or punch-drunk syndrome) or severe complications such as second-impact syndrome. At least six out of every 1000 people have a concussion each year. Common causes include sports injuries, bicycle accidents, auto accidents, and falls. Auto accidents and falls are the most frequent causes among adults. Concussion may be caused by a blow to the head, or by acceleration or deceleration forces without a direct impact. On such an impact the brain is bounced within the cranial cavity striking the side of the skull which sustains the blow and then, often, the other side on a rebound effect. The forces involved disrupt cellular processes in the brain for days or weeks. A debate about whether structural damage exists in concussion has raged for centuries and is still raging.