Spontaneous subarachnoid hemorrhage (SAH) is the leakage of arterial or venous blood into the subarachnoid space. Primary SAH may be caused by trauma, arteriovenous malformation, venous angioma, cavernous angioma, or bleeding from an intracranial tumor. Lesser causes of primary SAH include blood dyscrasias and CNS infections. Secondary SAH implies tracking of blood into the subarachnoid space from a primary intraparenchymal hemorrhage. Diagnosis of SAH is customarily made by CT and/or lumbar puncture. It is important to determine the etiology of the hemorrhage, and CT scanning, MRI imaging, and formal angiography are the mainstays of diagnosis in such cases. Treatment of SAH involves elimination of the offending lesion and differs depending on the etiology. Surgical intervention will almost always be proposed for cases of intracranial aneurysm and intracranial tumor, and in many cases of arteriovenous malformation hemorrhage. For SAH arising from trauma or from blood dyscrasias and infections, supportive management and medical treatment are most often the mainstays of therapy. Complications of SAH are greatest with aneurysmal subarachnoid hemorrhage and include vasospasm, rebleeding, and hydrocephalus. These problems are almost nonexistent with other etiologies of SAH. The treatment of vasospasm is with calcium channel blocking agents, hypertensive/hypervolemic therapy, and careful attention to surgical timing. Rebleeding may be prevented by use of antifibrin-olytic agents or early surgery. Hydrocephalus is treated by classical methods of ventricular drainage and shunting. The long-term outcome from primary SAH depends in large measure on the responsible lesion. Careful attention to surgical detail and optimal recognition and management of potential complications will clearly do much to improve the outcome for these unfortunate patients.