This chapter will outline the principles underlying Arterial Spin Labeling (ASL), discusses the issues confounding flow quantification and sensitivity, as well as methods developed to address these issues, and reviews some of the applications of ASL. In combination with routine magnetic resonance imaging (MRI) and magnetic resonance spectroscopy, ASL improved the sensitivity and specificity of grading gliomas. However, dynamic susceptibility contrast (DSC)-MRI remains the more commonly used technique because of superior signal-to-noise ratio (SNR) and the ability to acquire images of tumor hemodynamics and permeability. Disruptions in blood supply can cause a rapid cessation of synaptic activity and, if of sufficient severity, lead to cellular membrane failure. Likewise, increased synaptic activity requires a greater metabolic rate to maintain cellular ionic gradients, and this requires a concomitant increase in cerebral blood flow (CBF). Techniques for measuring absolute CBF can be broadly divided into several classifications depending on the characteristics of the flow tracer implemented.