An expert witness is a person who has knowledge, skill, experience or education in a particular field and is called upon to provide evidence in legal proceedings. They are often used in cases where complex technical or scientific issues need to be explained to a judge or jury. Expert witnesses can also testify on the basis of their opinion about a matter that requires specialized knowledge, such as medical malpractice or product liability cases. Their testimony is based on their expertise and not necessarily on personal observations of events in question.