Various functional imaging modalities can be applied in acute ischaemic stroke to identify functionally impaired, but morphologically preserved tissue (i.e. the penumbra), and to distinguish it from irreversibly damaged tissue. Flow thresholds for irreversible tissue destruction resulting in functional impairment, as determined by positron emission tomography, perfusion and diffusion-weighted magnetic resonance imaging, single-photon computed tomography and xenon computed tomography, were comparable and ranged between 5 and 12 ml/100 g per min for the lower and 14 and 22 ml/100 g per min for the upper limit of penumbra. These imaging modalities help to select patients for thrombolytic therapy and provide evidence for the effect of this treatment on critically perfused tissue. They can also serve as surrogate markers in the evaluation of therapeutic efficacy. Further progress in interventional neuroradiology has been achieved with intra-arterial thrombolysis, which has become a treatment option beyond the 3-h therapeutic window in acute ischaemic stroke. Angioplasty and stenting of stenosis of arteries that supply the brain with blood have reached a point in their development at which a randomized trial to compare these treatments with vascular surgery is warranted.