Rule 2:705.FACTS OR DATA USED IN TESTIMONY (Rule 2:705(a) derived from Code § 8.01-401.1).

Rule 2:705 governs when an expert’s supporting facts have to come out. In civil cases, subdivision (a) frees the expert from a disclosure requirement upfront: the expert may testify in terms of opinion or inference and give reasons for it without first disclosing the underlying facts or data, unless the court requires otherwise. That default lets direct examination move to the expert’s conclusion rather than walking through every underlying fact first.

The civil rule’s flexibility runs only one direction. Regardless of what happened on direct, the expert may in any event be required to disclose the underlying facts or data on cross-examination — the opposing party retains the ability to probe what the opinion rests on, even when direct examination skipped that step.

Criminal cases work differently. Subdivision (b) requires that the facts on which an expert may give an opinion be disclosed as part of the expert’s testimony, or set forth in a hypothetical question. Unlike the civil default, a criminal-case expert’s opinion cannot be announced without the jury also hearing — through the testimony itself or a hypothetical framing it — the facts the opinion is built on.