The non-invasive nature of arterial spin labelling (ASL) has opened a unique window into human brain function and perfusion physiology. High spatial and temporal resolution makes the technique very appealing not only for the diagnosis of vascular diseases, but also in basic neuroscience where the aim is to develop a more comprehensive picture of the physiological events accompanying neuronal activation. However, low signal-to-noise ratio and the complexity of flow quantification make ASL one of the more demanding disciplines within MRI. In this review, the theoretical background and main implementations of ASL are revisited. In particular, the perfusion quantification methods, including the problems and pitfalls involved, are thoroughly discussed in this article. Finally, a brief summary of applications is provided.