Cerebral microdialysis is now widely used as a bedside monitor of brain tissue biochemistry to identify cerebral hypoxia/ischaemia and assess cellular bioenergetics after brain injury. This chapter reviews the principles of cerebral microdialysis and identifies its role in detecting derangements of cerebral metabolism after brain injury. Microdialysis is used for a variety of clinical indications, including tissue monitoring in myocutaneous flap surgery, transplant surgery and bowel anastamoses. The concentration of substances in the dialysate will depend on the balance between substrate delivery to, and uptake from, the brain extracellular fluid (ECF) but also on several other factors. The pathophysiology of acute brain injury is complex, but two factors are of crucial importance: reduction of substrate delivery below critical thresholds, and the inability of brain cells to utilize delivered oxygen and glucose because of failing cellular metabolism.