Head injury is a trauma to the head, that may or may not include injury to the brain (see also brain injury). The incidence of head injury is 300 per 100 000 per year, with a mortality of 25/100 000 in North America and 9/100 000 in Britain.

Common causes of head injury are motor vehicle accidents (traffic accidents), occupational accidents, home accidents, falls and attacks. A head injury may cause a skull fracture, brain hemorrhage or a hematoma between the skull and the brain (subdural, subarachnoid or extradural hematoma). Common symptoms are loss of consciousness, drowsiness, double vision, seizures and headache. A fluid drainage from nose, mouth or ear is strongly indicative of the tearing of sheaths surrounding the brain, and can lead to secondary brain infection.

Especially in case of an extradural haematoma, symptoms may worsen after a temporary incline in general health. Typically it results from a blow to the side of the head. Patients are momentarily dazed or knocked out, followed by a period of relative lucidity which can last minutes or hours. Thereafter there is rapid decline as the blood collects, causing pressure on the brain. So, even if the patient is conscious, any head trauma should be regarded as a medical emergency. In case of a head trauma and loss of consciousness, first the person’s airway, breathing and circulation should be controlled (ABC of medical emergency) and then the head and neck should be stabilized and kept in line with the spinal cord. Attempts should be made to stop any bleeding by firmly pressing a clean cloth (if the bleeding wound is on a suspected skull fracture no pressure should be applied). The injured person should be evaluated with the Glasgow Coma Scale immediately after the injury, and at regular intervals if desired. This would aid in diagnosing the patient as having mild, moderate, or severe head injury.

Even people with slight head injuries, with no apparent signs or complaints, should be observed cautiously. During the first 24 hours after the incident, an observer can wake the victim every 2-3 hours and ask specific questions (e.g. his/her address or occupation, or today's date). In case of vomiting, drowsiness, personality change or severe headache, the victim should be transferred to a medical emergency unit. Mild headache and slight dizziness after a head injury is expected and does not necessarily require medical aid, if these symptoms do not persist.

Continuous head injuries (caused e.g. by boxing) can lead to a kind of dementia called Punch-drunk syndrome or to a Parkinson's Disease-like syndrome (dementia pugilistica) in the long run. A severe injury may lead to a coma, and eventually, death.

A closed (non-missile) head injury occurs when the head suddenly and violently hits an object, but the object does not break through the skull.

A penetrating (missile) head injury occurs when an object pierces the skull and enters the brain tissue.