When you need to prove a point about which you are not an authority, incorporating the voice of an expert into your speech can lend it some validity. Expert testimony is the opinion or judgment of an expert, a professional in his or her field. Opinions from doctors, coaches, engineers, and other qualified, licensed professionals serve as expert testimony. In a speech about knee surgery, for example, you might cite an orthopedic surgeon when explaining the difference between arthroscopy and knee replacement surgery. Lay testimony is the opinion of a nonexpert who has personal experience or witnessed an event related to your topic. In a speech on weather disasters, you could provide the testimony from a witness who survived a tornado.