Ultrasound is becoming increasingly important in medicine both as a diagnostic tool, as a therapeutic modality and surgery. Most medical applications of ultrasound are based on the properties of longitudinal waves in the frequency range 1-15 MHz. Ultrasonic waves travel at similar velocities (about 1500 m s−1) in most biological tissues and are absorbed in a rate of about 1 dB cm−1 MHz−1. Absorption occurs chiefly due to relaxation processes. It leads to thermal effects in biological systems. Mechanical effects, such as streaming and cavitations, are also important in certain situations, particularly at low frequencies. Highly focused ultrasound is used in neurosurgery; it is the only method for producing trackless damage deep in the brain. Diagnostic imaging using ultrasound finds applications in all tissues. Although ultrasound has been used in medicine since the 1930's, it is only recently that these techniques have been widely used and their potential fully recognized. Medical ultrasonics is now in a period of rapid growth and is on the verge of making a significant impact on clinical medicine. This field offers an open proving ground to many technologies developed for other applications, gives inspiration to the development of new technological advances, and provides a host of challenging and important problems that are unique to medicine and biology. The future of ultrasound in medicine depends upon talented people from medicine and the physical sciences working in close collaboration and upon the emergence of a new breed of research scientist trained in both medicine and engineering and dedicated not to the technology of destruction but rather to the preservation of life and humanity. The object of this paper is to present a review of the principles of medical ultrasonics and an introduction to a variety of its applications.