There are generally two types of witnesses whose testimony may be admissible in evidence in a criminal trial: (1) fact witnesses who have personal knowledge of facts relevant to the prosecution or defense of the pending criminal charge; and (2) expert witnesses, who have expert knowledge that may assist the jury in determining a fact issue.

Relevant evidence—including witness testimony—is generally admissible, but the court may exclude relevant evidence if its probative value is substantially outweighed by a danger of one or more of the following: unfair prejudice, confusing the issues, misleading the jury, undue delay, or needlessly presenting cumulative evidence.

A two-part test generally governs whether expert testimony is admissible: (1) the expert must be qualified (must be an expert on the subject) and (2) the testimony must be relevant and be based on a reliable foundation. The judge makes the initial determination about whether the expert and the expert’s testimony meet these requirements. The judge has broad discretion to determine the admissibility of expert testimony, and the judge’s decision to admit or exclude the expert testimony evidence will generally only be reversed on appeal if there is an abuse of the judge’s discretion.

In Virginia, as in other jurisdictions, witness testimony in criminal trials can be broadly categorized into two types: fact witnesses and expert witnesses. Fact witnesses provide testimony based on their personal knowledge of the events related to the criminal charge, while expert witnesses offer opinions based on their specialized knowledge, which can help the jury understand complex factual issues. The admissibility of evidence, including witness testimony, is subject to the court's discretion. Evidence must be relevant, but even relevant evidence can be excluded if it poses risks such as unfair prejudice, confusing the issues, misleading the jury, causing undue delay, or presenting cumulative information that is unnecessary. Expert testimony specifically must pass a two-part test to be deemed admissible: the expert must be qualified in the relevant field, and the testimony must be relevant and rest on a reliable foundation. The trial judge has considerable leeway in deciding whether an expert's testimony meets these criteria, and an appellate court will typically only overturn such a decision if it finds that the trial judge abused their discretion.