An expert is a person who testifies at a trial because she has special knowledge in a particular field that is not within the knowledge of the average person. There is no special certification for expert witnesses, but the more educated, credentialed, experienced and highly regarded they are in their field, the better their chances are of being qualified as an expert.The person qualified as an expert is allowed to testify about her opinion on the meaning of facts, which is an exception to the general rule barring opinion testimony. Non-expert witnesses are only permitted to testify about facts they observed and not their opinions about these facts. Some types of expert witnesses include: actuaries, who might testify about values of pension plans or businesses; child psychologists, who testify about the best interests of the child when custody or visitation is in dispute; and doctors who testify about the standard of care rendered to a plaintiff in a medical malpractice lawsuit.
If the expertise of a witness is challenged, the attorney for the party calling the "expert" must make a showing of the necessary background through questions in court, and the trial judge has discretion to qualify the witness or rule he or she is not an expert, or may only testify on limited subjects. Experts are usually highly paid for their services and may be asked about their compensation for testimony at trial. In most jurisdictions, the expert witness must be identified to the other party before trial for purposes of depositions and other discovery procedures, in order to gain advance knowledge of the basis of their testimony.