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 New Jersey, the admissibility of expert witness testimony is governed by the New Jersey Rules of Evidence, particularly Rule 702, which is similar to the Federal Rule of Evidence 702. An expert witness in New Jersey is someone who has specialized knowledge, skill, experience, training, or education that may help the court understand the evidence or determine a fact in issue. The key consideration is whether the expert's knowledge is relevant to the issues in the case and can provide assistance to the judge or jury. The determination of whether a witness qualifies as an expert and whether their testimony is admissible is made by the judge, who serves as a gatekeeper. The judge must ensure that the testimony is not only relevant but also reliable, based on a sufficient factual or data foundation, and that the expert has applied reliable principles and methods to the facts of the case. This gatekeeping function is informed by the New Jersey Supreme Court's adoption of the standards set forth in the landmark federal case Daubert v. Merrell Dow Pharmaceuticals, Inc., which requires that the methodology underlying the expert's testimony must be scientifically valid.