Harmonic imaging is introduced in the present article and its principle and physical characteristics is described in contrast to conventional ultrasonic imaging. The principle of the conventional image which uses ultrasonic echoes reflected at the interfaces between tissues is presented, and the nonlinear ultrasonic propagation which results in harmonic components is conceptually described. The pulse inversion technique which effectively extracts the harmonic components from the ultrasonic echo signals is introduced, and the advantages of the constructed harmonic images are summarized comparing with those of conventional ultrasonic images. The harmonic images are classified according to the mechanism of harmonic production, and the typical harmonic images obtained from patients are presented in contrast to the corresponding sonograms. Clinical significance and prospects of harmonic imaging and the future research areas are discussed.