The measurement of flow with magnetic resonance (MR) has been under study for well over 20 years. Early work considered means by which flowing spins could be tagged and then observed. From knowledge of the distance that the spins moved and the time of tagging and measurement, the velocity could be estimated. Since this early non-imaging work, the field of magnetic resonance imaging (MRI) has evolved. With the use of magnetic gradients it is possible to encode position with resonant frequency. Gradient pulses with zero net area have no effect on static spins but can be used to encode velocity. In fact this is one fundamental means by which flow is estimated quantitatively. Rather than use MRI to simply estimate velocity or flow in terms of a raw number, most modern MRI flow estimation techniques now provide a full image in which the image brightness is in some way directly related to flow on a pixel by pixel basis.