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 New York, witness testimony in criminal trials can be provided by two main types of witnesses: fact witnesses, who have direct knowledge of the events related to the case, and expert witnesses, who offer specialized knowledge to help the jury understand complex issues. The admissibility of evidence, including witness testimony, hinges on its relevance and the absence of significant negative factors such as unfair prejudice or potential to mislead the jury. For expert testimony to be admitted, it must pass a two-part test: the expert must be appropriately qualified in the relevant field, and the testimony must be relevant and rest on a reliable basis. The presiding judge has considerable discretion in determining whether an expert's testimony meets these criteria, and an appellate court will typically only overturn such a decision if it finds that the judge abused their discretion.