Tissue harmonic imaging is used on almost all modern diagnostic ultrasound equipment. The clinical success of this modality involves a combination of fortuitous factors. The improvement in lateral resolution obtained by transmitting on a fundamental frequency and receiving on a harmonic is well known. Other advantages include a deeper focal range with less absorption than would have been obtained by transmitting and receiving at twice the fundamental frequency. Natural apodization considerably reduces the 3-D volume of interaction with tissue. Simulations indicate that harmonic imaging is more robust in the presence of aberration. Analysis shows that the improvements over what would have been achieved by transmitting at twice the frequency are for shallower focal depths. Reasons for the noticeably enhanced contrast of harmonic imaging are discussed.