The physics of pulse NMR which is pertinent to an understanding of proton NMR imaging has been condensed and directed toward the medical physicist. The basic physical principals of spin manipulations using rf pulses are presented, and the relation between the quantum mechanical and the classical descriptions is covered in a rigorous fashion. The physics of relaxation is described and the relaxation times T1 and T2 are explained in some detail. Application of these spin manipulation techniques is illustrated by showing how they may be used in creating an image.