This paper deals with the specific diagnoses of the causes and mechanisms involved in the occlusion of the carotids or one of the major intracranial arteries. It is based on a series of 101 cases in which one or more of the above mentioned arteries were considered as occluded, the occlusion being of a pathologic origin. Artificial occlusion of the carotid and vertebral arteries as well as intracranial ones was also taken into account. Difficulties which may arise in every step of the differential diagnosis of arterial occlusion are pointed out, as are technical resources which help to avoid such difficulties. In order to render the report more practical the items concerned with angiographic diagnosis of occlusion of the intracranial and extracranial arteries of the brain are schematized as follows: 1) distinction between lasting and transitory non-pathologic occlusion; 2) distinction between organic and functional occlusion of an artery; 3) recognition of arterial narrowing; 4) distinction between congenital absence and acquired occlusion of an artery, specially one of the parts of the arterial circle of the base.