In this review, the principal flow effects in current MR imaging (MRI) techniques are presented. The first section is a general review of basic aspects of blood flow relevant to MRI and a brief summary of prior approaches to MR flow measurement and reported MRI flow effects. The second section contains a more detailed discussion of three basic underlying effects of flow that affect MRI: washout of saturated spins, displacement of excited spins, and phase shifts of excited spins due to motion along magnetic field gradients. The third section presents some experimental results to demonstrate the difference in washout effects observed with selective and nonselective refocusing pulses in spin-echo MRI. The final section considers some of the possible approaches to measurement flow with MRI.