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.

In Illinois, the admissibility of expert witness testimony is governed by the Illinois Rules of Evidence, which are similar to the Federal Rules of Evidence. An expert witness in Illinois is someone who has specialized knowledge, skill, experience, training, or education that can help the court understand the 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 a good fit for the subject matter of the case. The judge acts as a gatekeeper, ensuring that only relevant and reliable expert testimony is presented to the jury or considered by the judge. This gatekeeping function is crucial to prevent unreliable or irrelevant expert evidence from influencing the fact-finder. The judge will consider factors such as the expert's qualifications, the methodology used, and the applicability of the expert's approach to the facts of the case to determine whether the testimony is admissible.