Cerebral microdialysis (CMD) is a minimal-invasive monitoring technique for patients with subarachnoidal haemorrhage or severe traumatic brain injury, which allows the investigation of a wide spectrum of compounds in the brain tissue. The aim is a precocious identification of cerebral ischemia and secondary brain damage. The method was introduced in the early 1970s. By using commercial equipment it is nowadays possible to conduct on-line analysis at the bedside in the intensive care unit. The following article discusses the principles of CMD, the most commonly used biomarkers and the options during neurointensive care.