In the era of intravenous thrombolysis and intra-arterial thrombolysis/ thrombectomy a fast and accurate diagnosis of acute cerebral ischaemia is mandatory. Several requirements for imaging are relevant: 1) All acute stroke patients should be imaged and treated as fast as emergency trauma patients. 2) Exclusion of other causes of acute stroke. 3) Detection of early signs of acute ischaemic stroke. 4) Assessment of the type of ischaemic stroke. 5) Assessment of the severity and extent of cerebral blood flow impairment. 6) Evaluation of the vasculature for causes of ischaemic stroke. 7) Optimal logistics with continuous interaction between nurses, technicians, radiologists and neurologists. Both structural and functional imaging have to be performed. Both CT and MRI are adequate techniques to deal with the imaging requirements, although CT with CT perfusion and CT angiography has become the standard technique in most hospitals. Nevertheless, MRI has several advantages over CT and should be available as back-up or substitute for CT. Important questions are 1) can CT detect all non-ischaemic causes of stroke? 2) Can CT detect ischaemic stroke in the early phase and make a distinction between the different types of ischaemic stroke? 3) How to interpret CT-and MRI-perfusion images? 4) How to detect distal occlusions? 5) How to differentiate between real and pseudo-occlusions of the internal carotid artery? Finally, a systematic approach in the analysis of the acquired images will facilitate a fast and accurate contribution of radiology to the optimal treatment of patients with acute ischaemic stroke. Stroke has gained acceptance as a treatable disease due to new developments both in diagnostic and therapeutic tools. The aims of therapy in stroke are to reperfuse hypoperfused areas and to open occluded vessels in the presence of a neurological deficit. Initially after the occlusion there is a sudden decrease in local blood flow. After a sufficient decrease, there is at first a reduction in function that is still associated with viability: this area where the thresholds for irreversible ischemia have not yet been attained is called the penumbra. While initially this was an area with reduced electric function but retained viability and membrane integrity, this area has been defined by diminished blood flow. Additionally a "core" infarct can be detected that is constituted of irreversibly ischaemic tissue. On MRI the penumbra was first delineated as the so-called diffusion-perfusion mismatch. CTbased algorithms also allow defining areas of reduced flow that are not yet necrotic. The ischaemic core can be both seen on DWI MR images or on unenhanced CT images: the addition of perfusion CT and perfusion MR techniques allows us to refine the visualisation of what should be the therapeutic target: the penumbra, since its reversibility should restore function.