Nuclear acoustic resonance (NAR), like nuclear magnetic resonance (NMR), can be used as a spectroscopic imaging tool to detect and characterize soft tissue densities and differences on the atomic scale. Whereas NMR uses electromagnetic radiation to induce energy level transitions, NAR uses acoustic radiation. The frequency of this radiation is typically 1 to 100 MHz; NAR imaging therefore uses ultrasonic energy to induce transitions among the nuclear spin energy levels. By means of piezoelectric transducers, polarized acoustic waves are generated and propagated within a specimen. If these perturbations are in resonance with the specimen's nuclear spin system, then the acoustic waves will periodically modulate an internal magnetic dipole or electric quadrupole interaction as acoustic energy is absorbed. The measurement of this acoustic energy absorption is analogous to the computation of the spin-lattice relaxation time, T1, caused by the release of radiofrequency energy into the surrounding lattice of an excited nucleus and used in magnetic resonance imaging. Accordingly, NAR imaging combines the tools of ultrasound with the techniques of MRI to yield a new and potentially valuable medical imaging modality. The purpose of this paper is to discuss the essential physics of NAR, and to suggest how NAR signals can be processed for medical imaging.