The modern obstetrician and gynaecologist is increasingly using a wide range of equipment and instruments in their clinical practice, for both diagnostic and therapeutic purposes. With the introduction of the new curriculum, a basic understanding and use of ultrasound imaging are mandatory for all, in addition to a knowledge of conventional diagnostic and therapeutic techniques. However, not all practitioners fully appreciate the background principles on which these techniques are based. It is not so much the complex knowledge of physics and formulae that is required, but rather a knowledge of how the principles of physics are applied in clinical practice. This is fundamental to a better understanding of the techniques as well as their safe, optimal and efficient use. This chapter will consider the scientific foundations and uses of a procedure frequently encountered in obstetric and gynaecological practice.