Aneurysmal subarachnoid hemorrhage (SAH) is a rare but life-threatening type of stroke with high mortality. Modern imaging techniques such as computed tomography (CT), CT angiography (CTA) and digital subtraction angiography (DSA) nowadays enable a rapid and reliable diagnosis. In unclear cases, cerebrospinal fluid analysis is an essential component. This first part of the CME article focuses on the pathophysiology, epidemiology and clinical presentation of SAH. In addition to standard diagnostic procedures, special forms such as perimesencephalic SAH and SAH due to intradural dissection are discussed. The goal is to provide a solid basis for the differential diagnostic classification and further clinical management.