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 Missouri, expert witness testimony is governed by Missouri Revised Statutes and case law, which align with the principles outlined in the Federal Rules of Evidence. An expert witness in Missouri is someone who has specialized knowledge, skill, experience, training, or education that can help the court understand evidence or determine a fact in question. The decision to qualify someone as an expert witness is at the discretion of the presiding judge, who evaluates whether the expert's knowledge is relevant to the case and whether their testimony is reliable. The judge acts as a gatekeeper, ensuring that only reliable expert testimony is presented to the jury or considered by the judge in a bench trial. This gatekeeping function is crucial to prevent unreliable or irrelevant expert testimony from influencing the outcome of a trial. The criteria for admitting expert testimony in Missouri do not follow a strict formula, but rather require a case-by-case assessment of the expert's qualifications and the applicability of their expertise to the specific issues at hand.