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 Pennsylvania, witness testimony in criminal trials can be provided by two main types of witnesses: fact witnesses and expert witnesses. Fact witnesses offer testimony based on their personal knowledge of the facts surrounding the case, while expert witnesses provide 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 assessment of its relevance and potential prejudicial impact. Evidence may be excluded if it poses risks such as unfair prejudice, confusion, misleading the jury, undue delay, or presenting redundant information. Expert testimony must pass a two-part test to be admitted: the expert must be qualified in the relevant field, and their testimony must be relevant, reliable, and based on a solid foundation. The presiding judge has considerable discretion in determining the admissibility of expert testimony, and appellate courts typically only overturn such decisions if there is evidence of an abuse of discretion by the judge.