Ultrasound has been used safely in the forefront of medicine and surgery for decades, helped by the development of increasingly sophisticated and miniaturized machines. As an ultrasound scan is relatively cheap and does not use ionizing radiation, it remains a first-line imaging tool in most aspects of general surgery and increasingly within emergency departments and other non-radiological clinical settings. Interpretation of ultrasound images is heavily operator dependent with pathology-mimicking image artefacts creating many pitfalls for the untrained operator. Ultrasound guidance for biopsies is now standard practice giving improved accuracy and yields. In this chapter, basic physical principles are discussed followed by descriptions of some of the many applications of ultrasound to surgical practice with strengths and weaknesses highlighted.