The physics, instrumentation, and general aspects of nuclear magnetic resonance (NMR) imaging are discussed. NMR images are constructed from magnetic signals emitted by certain atomic nuclei when they are subjected to simultaneously applied magnetic fields and externally generated radiofrequency (r.f.) energy. Hydrogen (the nucleus of which is a single proton) is sensitive to NMR and sufficiently abundant in tissues to produce the signals necessary for image formation. The NMR imaging device consists of a large magnet, a radiofrequency-transmitter coil, and computer hardware. Following r.f. pulsing, the strength and origin of the magnetic signals can be determined by magnetic field gradients that are superimposed on a magnetic field. Then this spatial information can be encoded electronically and reconstructed into a cross-sectional image. NMR imaging is valuable for the diagnosis of a variety of diseases and traumas since a high level of lesion and soft tissue contrast is possible in most types of pathology. Compounds containing paramagnetic elements (magnetopharmaceuticals) may be useful to enhance tissue contrast and organ detail. The high level of tissue detail, sensitivity to pathology, and apparent lack of hazard make NMR an important addition to the existing diagnostic modalities.