Ultrasound therapy involves production of longitudinal mechanical waves above the audible range (20 KHz). The frequencies used in physiotherapy vary from 0.75 MHz to 3 MHz. These are produced by distortion of quartz crystal, by a high frequency alternating current. These sound waves require a conducting medium like oil or water to reach from the probe head to tissues. The probe head should be applied perpendicular to tissues to present refraction and it should be moved continuously to prevent concentration on one tissue area. Higher the frequency, greater the absorption and smaller the depth of penetration.