To apply ultrasound energy to the human body for the purpose of creating a surgical effect, it is imperative that the basic laws of nature that govern the interaction of sound on various media be clearly understood; otherwise, the operator will fail to appreciate the mechanism by which the desired result can be best obtained. This article reviews the physical principle, tissue interactions, wave properties, equipment, and biologic and micromechanical effects of ultrasound-assisted lipoplasty. Ultimately, the technique is based on the perfect control and balance between the thermal effect and cavitational phenomenon.