Cerebral microdialysis is a well-established laboratory tool that is increasingly used as a bedside monitor to provide on-line analysis of brain tissue biochemistry during neurointensive care. This review describes the principles of cerebral microdialysis and the rationale for its use in the clinical setting, including discussion of the most commonly used microdialysis biomarkers of acute brain injury. Potential clinical applications are reviewed and future research applications identified. Microdialysis has the potential to become an established part of mainstream multi-modality monitoring during the management of acute brain injury but at present should be considered a research tool for use in specialist centres. The principles of MD have been reviewed in detail elsewhere but a brief summary is provided here. A MD catheter consists of a fine double lumen probe, lined at its tip with a semi-permeable dialysis membrane. The probe tip is placed into biological tissue and perfused via an inlet tube with fluid isotonic to the tissue interstitium. The perfusate passes along the membrane before exiting via outlet tubing into a collecting chamber Diffusion drives the passage of molecules across the membrane along their concentration gradient. Molecules at high concentration in the brain ECF pass into the perfusate with minimum passage of water and, as the perfusate flows and is removed at a constant rate, the concentration gradient is maintained.