Arachnoid cysts are an entirely benign condition characterised by an abnormal collection of the normal cerebrospinal fluid. Cerebrospinal fluid is the liquid in which the brain and spinal cord floats. Therefore, the arachnoid is a meningeal layer that contains the entire brain and spinal cord. An arachnoid cyst develops when some of this arachnoid encloses a collection of CSF that cannot communicate with the rest of the brain and spinal cord and, therefore, produces a mass lined by the arachnoid and containing a sealed off area of CSF. Often, an arachnoid cyst is diagnosed on CT or MR scan of the brain and is of no clinical significance in that it is simply present but not causing any symptoms and will not do so throughout the person’s life. Rarely, the mass produced by the arachnoid cyst can press on some of the structures of the brain and produce symptoms related thereto. The symptoms that it may produce, therefore, are entirely related to the area of the brain it presses upon. We would wish to emphasize that in the vast majority of cases, an arachnoid cyst is a diagnosis made by the radiologists after a scan and no treatment is required but the condition should be discussed with a neurosurgeon with expertise in the condition. Having consulted such a neurosurgeon, most people can be reassured and no further follow up is required.