A concussion is a brain injury that occurs when you receive a sudden blow or jolt to the head. It is a type of traumatic brain injury (TBI). Your doctor may call a concussion a "mild brain injury." Most concussions are not life- threatening, but they can have serious effects. A concussion may lead to thinking, sleeping, and balance problems, and symptoms. In rare cases, a blood clot may form on the brain after a concussion. This is a life-threatening condition. Warning signs include persistent headache, extreme drowsiness, slurred speech, and repeated vomiting or nausea. Follow the links below to find WebMD's comprehensive coverage about concussion, how it develops, what the symptoms are, how to treat it, and much more.