Dr. Robert Grant of the Department of Clinical Neurosciences in Edinburgh, Scotland, reviews the etiology, presentation, investigation, and treatment of arachnoid cysts in the brain and spine. Arachnoid cysts are common incidental findings (1.4% of adults and 2.5% of children having an MRI scan). Asymptomatic cysts should be left alone. Some are associated with seizures, local pressure effects, or hydrocephalus. Urine should be checked for glutaric acid in cases of bitemporal arachnoid cysts as glutaric aciduria type 1 is present in 25% of cases. Symptomatic cysts will require intervention in the presence of hydrocephalus or spinal cord compression. Operation to treat seizures or headache is successful in only 33% to 50% of cases. Electroencephalographic seizure focus coinciding with temporal arachnoid cyst and SPECT imaging demonstrating impaired cerebral perfusion may be helpful when considering intervention. Minimally invasive neuroendoscopic techniques are being more widely practiced where there is hydrocephalus or suprasellar cysts, but resection and marsupialization for large Sylvian fissure and cortical arachnoid cysts has superior success rates to neuroendoscopic procedures. Endoscopic fenestration, although less effective at reducing cyst size, has a lower complication rate. The complication rate of fenestration of arachnoid cysts in pediatric cases is over 35%. About 10% of arachnoid cysts recur after fenestration, and subdural hygromas requiring re-operation occur in 6% of all cases of large Sylvian fissure arachnoid cysts.