Vasospasm is one of the most frequent complications of subarachnoid hemorrhage (SAH) and causes high mortality rates and serious neurologic deficits, resulting in invalidity. Diagnosis is based on precise clinical criteria (onset between the 3rd and the 8th day after the SAH) and is confirmed by angiographic examination. The etiology certainly involves many factors, though the identification of the primary cause (conductive to morpho-structural alterations of the artery walls) seems to be troublesome. Experimental evidence indicates that the basis of the reactions which cause the onset of the vasospasm lies in the blood which pools in the subarachnoid zone. The authors summarize the main pathogenetic theories, especially as far as the initial biochemical sequence is concerned. Among the numerous classes of substances involved in the genesis of vasospasms, a relevant role is played by oxyhaemoglobin, free radicals, lipidic peroxides and by the metabolites of arachidonic acid. The main purpose of vasospasm treatment is the possibility to eliminate and prevent ischemic complications. Early surgical exclusion of the aneurysm causing the bleeding allows for the removal of peri-aneurysmatic blood clots. Pharmacological treatment employs numerous classes of substances: among these calcium antagonists presently appear to give satisfactory results.