A witness who qualifies as an expert because of knowledge, skill, experience, training, or education may testify as an expert if scientific, technical, or other specialized knowledge will assist the judge, jury, or other factfinder to understand the evidence or determine a contested fact issue.

But there is no rigid formula for determining whether a particular witness is qualified to testify as an expert. Generally there must be a fit between the subject matter at issue and the expert witness’s familiarity with the subject matter. And this determination is made by the judge, who acts as a gatekeeper to only allow reliable expert witness testimony to be heard and considered by the judge or jury—and to exclude such testimony when it is not reliable.

In California, expert witness testimony is governed by both state statutes and case law. Under California law, an expert witness is someone who has special knowledge, skill, experience, training, or education that makes them qualified to provide testimony on matters that require expertise beyond that of an average person. The role of the expert is to help the judge or jury understand complex evidence or to determine a fact that is in dispute. The determination of whether a witness qualifies as an expert is not based on a strict formula but rather on whether there is an appropriate fit between the witness's expertise and the subject matter of the case. The judge serves as a gatekeeper, assessing the reliability of the expert's testimony and deciding whether it should be admitted for consideration. This gatekeeping function is crucial to ensure that only relevant and reliable expert testimony is presented, in line with the standards set forth in the California Evidence Code and interpreted by case law, such as the landmark decision in People v. Kelly and the adoption of aspects of the federal Daubert standard.