In the Western world, trauma is the commonest cause of death in the under-40s, and, head injury is the leading component of this. Traumatic brain injury (TBI) is not a single disease, but a number of pathologies that can occur in isolation (e.g. an extradural haematoma) or co-exist (a subdural haematoma with diffuse axonal injury). TBI has important social, emotional and financial consequences, both for the individual, their families and society. Despite this, there is a great deal we do not know about brain injuries and how best to manage them. This article outlines the fundamental principles of pathophysiology, diagnosis and management.