The spatially selective radiofrequency (RF) excitation and inversion capabilities of magnetic resonance make possible a totally non-invasive method for perfusion imaging, which is known as arterial spin labelling (ASL). The clearest distinction between labelling approaches is between continuous ASL and pulsed ASL. Continuous ASL usually employs a special RF labelling scheme known as flow driven adiabatic inversion. ASL perfusion measurement differs from dynamic susceptibility contrast (DSC) imaging in several important ways. Typically imaging of perfusion with ASL requires multiple averages to achieve an acceptable signal-to-noise ratio. ASL methods have been validated against existing methods for quantifying cerebral perfusion. CBF values obtained using continuous ASL has been validated against microspheres in a rat model of middle cerebral artery occlusion and against 15O-PET scanning in humans. Measurement of regional CBF is likely to have several other applications in cerebrovascular disease and stroke.