Acquired brain injury (ABI) is brain damage caused by events after birth, rather than as part of a genetic or congenital disorder such as fetal alcohol syndrome, perinatal illness or perinatal hypoxia. ABI can result in cognitive, physical, emotional, or behavioural impairments that lead to permanent or temporary changes in functioning. These impairments result from either traumatic brain injury (e.g. physical trauma due to accidents, assaults, neurosurgery, head injury etc.) or nontraumatic injury derived from either an internal or external source (e.g. stroke, brain tumours, infection, poisoning, hypoxia, ischemia, encephalopathy or substance abuse). ABI does not include damage to the brain resulting from neurodegenerative disorders.

While research has demonstrated that thinking and behavior may be altered in virtually all forms of ABI, brain injury is itself a very complex phenomenon having dramatically varied effects. No two persons can expect the same outcome or resulting difficulties. The brain controls every part of human life: physical, intellectual, behavioral, social and emotional. When the brain is damaged, some part of a person's life will be adversely affected.

Consequences of ABI often require a major life adjustment around the person's new circumstances, and making that adjustment is a critical factor in recovery and rehabilitation. While the outcome of a given injury depends largely upon the nature and severity of the injury itself, appropriate treatment plays a vital role in determining the level of recovery.