The flow of water in porous materials has been visualized using nuclear magnetic resonance imaging (MRI). For flow in an initially dry bed, the water gives a large signal that can be detected directly. Flow in a wet bed is visualized indirectly by displacing the pure water with a dilute solution of paramagnetic ions. This solution does not give an MRI signal and so can be contrasted with pure water. Another use of MRI is to observe the absorption of water by a solid. The MRI technique is sensitive and can give accurate and quantitative results for flow with low Peclet number.