Background: Sound travels in waves that transport energy from one location to another. Ultrasound is the name given to sound waves that have frequencies greater than 20000Hz. It's too high pitched for human hearing, but many animals, such as dogs, cats and bats can hear ultrasound. As the ultrasound waves travel through tissues, they are partly transmitted to deeper structures, partly reflected back to the transducer as echoes, partly scattered, and partly transformed to heat. The amount of echo returned after hitting a tissue interface is determined by a tissue property called acoustic impedance which is an intrinsic physical property of a medium defined as the density of the medium times the velocity of ultrasound wave propagation in the medium. Physiologic effect of US: The thermal therapeutic ultrasound include increased tissue temperature, hyperdynamic tissue metabolism, increased local blood flow, increased extensibility of collagen fibers, and reduced viscosity of fluid elements in the tissue. The nonthermal mechanisms include ultrasonic cavitation, gas body activation and mechanical stress or frequency resonance nonthermal processes. Therapeutic effects of US: Therapeutic ultrasound is delivered in two modes; the continuous mode in which the delivery of ultrasound is non-stop throughout the treatment period and the pulsed mode in which the delivery of ultrasound is intermittently interrupted. Essential treatment parameters for therapeutic ultrasound include frequency, intensity, treatment mode, treatment time. Clinical applications: US in damaged muscle accelerates the repair process due to the decrease in the response and number of inflammatory cells and increases the proliferation and differentiation of muscle cell lines together with the formation of the connective tissue, improving mechanical resistance in the early stages. US is used also in chronic pain syndrome, tissue repair and wound healing and extra and intracorporeal shock wave lithotripsy. Cancer therapy: High-intensity focused ultrasound is a noninvasive therapy that makes entire coagulative necrosis of a tumor in deep tissue. HIFU ablation can destroy all proliferating tumor cells and their feeding blood vessels at the same time; this may break interdependent vicious cycle of tumor angiogenesis and tumor growth. Conclusion: Applications of ultrasound in medicine for therapeutic purposes have been an accepted and beneficial use of ultrasonic biological effects for many years. While therapeutic ultrasound is safe for treating many clinical conditions, it may cause substantial bioeffects and patients should be fully informed of possible benefits and risks.