Brain damage or brain injury is the destruction or degeneration of brain cells.

Brain damage may occur due to a wide range of conditions, illnesses or injuries. Possible causes of widespread (diffuse) brain damage include prolonged hypoxia (shortage of oxygen), poisoning, infection and neurological illness. Common causes of focal or localized brain damage are physical trauma (head injury), stroke, aneurysm or neurological illness.

The extent and effect of brain injury is often assessed by the use of neurological examination, brain imaging and neuropsychological assessment.

Brain injury does not necessarily result in long-term impairment or disability, although the location and extent of damage has a significant effect on the likely outcome. In serious cases of brain injury, the result can be permanent disability, including neurocognitive deficits, speech or movement problems and mental handicap. Severe brain damage may result in persistent vegetative state, coma, or death.

Various professions may be involved in the medical care and rehabilitation of someone who suffers impairment after brain damage. Neurologists, Neurosurgeons and Physiatrists are doctors who specialise in treating brain injury. Neuropsychologists (especially clinical neuropsychologists) are psychologists who specialise in understanding the effects of brain injury and may be involved in assessing the extent of brain damage or creating rehabilitation programmes. Occupational therapists may be involved in running rehabilitation programmes to help restore lost function or help re-learn essential skills.

Because the brain continues to develop until the late teens, damage sustained during childhood is often recovered from more successfully than if the same injury were to occur in adult life.

The effects of impairment or disability resulting from brain injury may be treated by a number of methods, including medication, psychotherapy, neuropsychological rehabilitation, surgery or physical implants such as deep brain stimulation.