A new technique has been devised to measure the velocity of fluid motion within the human body by using Nuclear Magnetic Resonance (NMR) Imaging. The phase of the NMR signal is modulated in the presence of motion by the time varying magnetic field gradients used in the imaging process. The magnetic field gradient pulse waveforms can be tailored to produce phase modulations which are linearly dependent on the components of How velocity in a particular direction. However phase modulations are also caused by inhomogeneities in the main magnetic field, and by chemical shift and magnetic susceptibility effects, as well as by flow velocity components in other directions. In order to exclude these unwanted phase modulations a novel technique has been developed in which two phase images are acquired, each having a different velocity dependence. Subtraction of these two images eliminates the phase modulations common to both images and leaves only the velocity-dependent phase modulations corresponding to the direction of flow being measured. The velocity dependence in each image is controlled by varying the temporal separation between two of the gradient pulses. The phase images are acquired using a field echo pulse sequence in order to maximise the signal from fast flowing blood and the two acquisitions are interleaved to avoid the possibility of mis-registration when the two phase images are subtracted. This flow imaging technique has been implemented on a 0.08 Tesla NMR imaging system (M& D Technology Ltd). It is possible to measure flow velocity components both within the imaged slice and perpendicular to the slice. The slice orientation can be angled by up to 30 degrees to allow imaging of vessels which run obliquely through the body. Measurement of velocity components perpendicular to the imaged slice gives the most accurate results whilst imaging of velocity components within the imaged slice is of use for visualisation of flow and positioning of subsequent acquisitions. An clcctro-cardiographic gating system has been developed to synchronise the NMR imaging system with the subject's cardiac cycle. It is thus possible for multiple images to be acquired throughout the cardiac cycle at the same spatial position, enabling the study of pulsatile flow. Further development of the magnetic field gradient pulse waveforms has been carried out to reduce signal losses caused by accidental phase modulations in the presence of high velocity gradients. The NMR flow measurement technique has been calibrated for both low velocity (15cm/s maximum) and high velocity (lOOcm/s maximum) ranges. There is excellent agreement between the calibrated velocity/phase relationships and those predicted by theory. The NMR technique has been compared with Doppler ultrasound in the measurement of velocity waveforms in the internal and common carotid arteries of healthy volunteers. The maximum velocities obtained from the NMR technique were compared and correlated well with those obtained from Doppler ultrasound. The NMR technique has also been used to investigate the low velocity pulsatile motion of cerebrospinal fluid related to the cardiac cycle in healthy volunteers and in one patient with syringomyelia.