Specific restrictions are placed on what an expert witness can and cannot testify to. Expert witnesses are permitted to give their opinions only on matters for which they are deemed qualified as having expertise. This distinction is very important in circumscribing what is often heavily weighted testimony.
Expert witnesses are a specific exception to the “hearsay rule.” Traditional witnesses can testify only to what they know and have experienced and are not permitted to speculate about hypotheticals or report third-party testimony. Expert witnesses, however, may discuss hypotheticals and other “hearsay” matters, as long as those matters are within the purview of their expertise. For example, an expert witness who is a surgeon can discuss hypotheticals within a medical malpractice case, including background and contextual information, as long as their testimony does not extend beyond that subject.
That said, expert witnesses must take an oath to testify truthfully and obey all court rules and procedures, such as respecting the “chain of custody” when handling and examining evidence.