Magnetic resonance imaging (MRI) is emerging as a measurement tool with unique capabilities in the field of reaction engineering-in particular, the ability to study three-dimensional, optically opaque systems, and to quantify the physics and chemistry that is occurring at multiple length-scales within such systems. Here we highlight recent developments in developing MRI methods for application to studying both hydrodynamics and chemical conversion in gas-liquid-solid reactors. Examples are taken from parallel channel, structured reactors and fixed beds. In the former case it is now possible to image liquid velocities up to similar to 10 ms(-1) within individual channels within a ceramic monolith. In the context of fixed beds, the ability of MRI to reveal the mechanism of the trickle-to-pulse transition is reviewed. A new application of MRI to image the changing liquid holdup between individual packing elements within a bed during periodic operation is then presented. Finally, the state-of-the-art in mapping chemical conversion in trickle-bed reactors is discussed. (C) 2007 Published by Elsevier Ltd.