Extravasation and subsequent accumulation of blood in the subdural space between the dural and arachnoidal layers of the MENINGES. This condition primarily occurs over the surface of a cerebral hemisphere (HEMATOMA, SUBDURAL, INTRACRANIAL), but may develop in the posterior fossa and spinal canal (HEMATOMA, SUBDURAL, SPINAL). Acute (HEMATOMA, SUBDURAL, ACUTE) and chronic (HEMATOMA, SUBDURAL, CHRONIC) forms exist.