We often use the terms “concussion” and “mild” or “minor” head or brain injury, synonymously; and read “moderate”, “severe” and, more recently, “very severe” brain injury in medical records. When used appropriately these terms convey information about the clinical presentation, especially in the hours and days following a brain injury. This information helps us to predict overall outcomes, and informs opinions on prognosis. The most widely used definitions are shown in table A (below). Sometimes it is more useful to consider a TBI according to anatomy and pathology. To do this we must first understand some basic principles of brain structure and function.