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 Maryland, expert witness testimony is governed by Maryland Rule 5-702, which aligns with the principles described. An expert witness, due to their knowledge, skill, experience, training, or education, may provide testimony if it offers helpful scientific, technical, or specialized knowledge to the judge or jury in understanding the evidence or deciding upon a contested fact. The qualification of an expert is not determined by a strict formula but rather by the relevance and reliability of their expertise to the subject matter at hand. The judge serves as a gatekeeper, assessing the admissibility of the expert testimony to ensure that it is pertinent and reliable. The judge's role is to admit expert testimony that will aid the fact-finding process and to exclude it when it does not meet the standards of relevance and reliability.