Brain oedema leads to pathological changes in intracranial pressure (ICP) and cerebral blood flow in a wide range of clinical scenarios, because the brain produces oedema in response to many diseases. Clinical management often focuses on minimizing elevations of ICP and maintaining adequate cerebral blood flow. A working knowledge of physiological principles linking brain oedema, ICP and blood flow is essential for clinicians facing these clinical problems. These principles are explained here, and also some insights are provided concerning the mechanisms of disease on the cellular level. This is then applied to the clinical problem of traumatic brain injury illustrating physiological principles in clinical practice.